Contemporary built environments are full of misalignments, or mismatches, of the physical environment with the needs of individuals and groups that have evolved over the history of our species (e.g., Lloyd et al. 2011). Many of these mismatches could be avoided if the AEC industry operated more directly within an evolutionary framework (Harmon 2018a), including the design/construction process itself, the development and application of equipment, building materials, and codes/standards, and the conducting of research underlying many of the industry’s current normative practices. The development and marketing of germicidal ambient lighting is one potential example of this currently unfolding.

The use of UV light (between a wavelength of 10nm and 400nm) in HVAC systems at the cooling coils, drain pans, etc. to prevent the growth of mold and harmful bacteria has been common practice for some time. And some facilities, such as hospitals, use portable UV lighting units at these lower wavelengths to disinfect whole spaces. This is done during unoccupied conditions because UV light is harmful to humans, though the specifics depend on which part of the spectrum is being used, the duration of exposure, and what specific health impacts are being considered (i.e., skin cancer, cataracts, immune system disorders, etc.).

Now several manufacturers are producing ambient germicidal luminaires/systems using the lower end of the visible spectrum (violet and blue light) to provide disinfecting capabilities in both occupied and unoccupied conditions. Given the growing challenges our world faces with respect to antibiotic resistance, the spread of infectious diseases, and food contamination, all of which will be exacerbated by climate change, this evolution of germicidal lighting certainly offers potential benefits.

The technology makes use of wavelengths within the 400nm – 450nm visible spectrum range to disinfect and kill bacteria. This narrow germicidal range may be blended with overall white light to function as the primary source of ambient light, or it may be employed alone in fixtures at a higher output during unoccupied conditions, such as overnight, to maximize disinfection. Or both modes may be incorporated into a single luminaire. Some manufacturers are also marketing these luminaires for use outside of more specialized settings, like hospital surgical suites or cleanrooms, to a wider array of space types, ranging from school classrooms and daycare to hospitality settings and dormitory rooms.

While such luminaires do offer benefits, there is the possibility that their widespread deployment could have unintended health consequences. It could create environments misaligned, or mismatched, with human physiological needs that have evolved over the history of our species (e.g., Lloyd et al. 2011). An evolutionary mismatch occurs when traits (T) that are adaptable in an “ancestral” environment (E1) end up being maladaptive in another environment (E2) (Lloyd et al. 2011; Wilson 2018; Wilson 2019). The new dysfunctional nature of the traits can be defined strictly in terms of evolutionary fitness or also refer to general health and wellness (Lloyd et al. 2011).

One example of this is the complex of traits making up our circadian system, which helps regulate our biological clock. This system evolved over the course of our evolutionary history spent primarily outside (E1), adapting to and making use of the environmental cues of the day/night cycle. Now we spend over 90% of our time inside (Klepeis et al. 2001), under electric illumination with spectrums and intensity levels that vary from what we experience outdoors, often extending well into the night (E2). Our circadian system isn’t adapted to function as well in this “new” environment, with negative health consequences as a result (Bodreau et al. 2013). These negative impacts are further compounded by our exposure to the illuminated screens of our electronic devices.

Further illumination is provided through Tinbergen’s (1963) discussion of traits relative to their 1) functional basis, 2) mechanistic basis, 3) development during the organism’s (or group’s) lifetime, and 4) heritable history (Lloyd et al. 2011; Wilson 2018; Wilson 2019). In the case of the circadian system, its function refers to the ultimate reason for its evolution – to help regulate the human biological clock. The mechanisms of the circadian system refer to how it physically works in humans – how the patterns of light and dark that reach the back of the eyes are converted to neural signals that help synchronize our “biological clocks” with the local time (Figueiro et al. 2016). The development of the circadian system over the course of an organism’s lifetime may indicate that it operates differently at different stages of human development or is impacted by the environment differently. Specific cues in E1 may actually impact how it develops and how successfully it operates. And finally, its heritable history refers to how the circadian system evolved over the history of our species, and our species’ ancestors, and how it compares to the evolution of similar systems in other species.

One final useful layer of examination is Mayr’s (1961) distinction between ultimate and proximate causation (e.g. Lloyd et al. 2011; Wilson 2018; Wilson 2019). Explanations of ultimate causation explain why a trait exists (usually as a result of evolutionary forces) compared to other potential traits that could have performed the same function. These are Tinbergen’s functional and history questions (Lloyd et al. 2011). Could another suite of traits have functioned in the same manner as the circadian system had our evolutionary history been different? Explanations of proximate causation look at the specific mechanisms of traits and how those mechanisms develop over the course of an organism’s life – these are Tinbergen’s mechanistic and development questions (Lloyd et al. 2011).

Framing issues like circadian lighting design or the use of germicidal ambient lighting within an evolutionary framework that a) distinguishes between ultimate and proximate causation, b) accounts for Timbergen’s four characteristics of traits, and c) recognizes the potential for evolutionary mismatches will help ensure the following:

  • The right questions are being asked.
  • Research is structured in an appropriate manner that recognizes, and is capable of uncovering, evolutionary mismatches as well as determining the potential short and long-term negative impacts of such mismatches.
  • Environments are designed/constructed in a manner that avoids or minimizes mismatches, including the selection and installation of equipment, materials and furnishings.

After digging into this, specific issues that BranchPattern’s Research & Development team sees as requiring additional research, preferably within an evolutionary framework making use of the above discussion, include the potential development of tolerant or resistant bacteria, potential negative impacts on bacteria outside of the human body, and potential negative impacts on human physiology. I’ll address each of these areas by looking at five questions covering these issues. But first let’s look at the technology’s effectiveness as a disinfectant.

Effectiveness as a Germicidal Agent

For simplicity sake, I will follow the lead of some researchers and use “blue light” to refer to the wavelengths between 400nm and 450nm. The peer reviewed literature and researchers I’ve contacted (Maurice Gallagher, personal communication, February 20, 2019; Martin Hessling, personal communication, February 22, 2019) generally indicate that ambient luminaires using blue light can function as a disinfectant. However, germicidal effectiveness depends on the microbial levels present, the types of bacteria present, and the specific environmental context, including the specific blue light wavelengths used, their intensity, and the time of exposure (Dai et al. 2012a; Endarko et al. 2012; Gwynne and Gallagher 2018; Halstead et al. 2019; Hessling et al. 2017; Maclean et al. 2014; Murrell et al. 2018; Ramakrishnan et al. 2014; Wang and Dai 2017; Wang et al. 2017; Zhang et al. 2016).

In essence the effectiveness depends on Timbergen’s four trait characteristics relative to the different types of bacteria being considered and the specific environments they inhabit that are subject to sterilization. The varying histories of different types of bacteria have resulted in different mechanisms that function to provide tolerance or resistance against various germicidal agents. How effective these mechanisms are will depend on how adapted they are to the specific environmental factors at play, including the characteristics of the blue light itself. How these mechanisms develop over the course of a bacteria’s life, also impacted by the bacteria’s history, will also affect its ability to resist or tolerate germicidal agents under various environmental conditions. Introducing germicidal blue light essentially creates a new environment (E2) that most bacteria appear to be mismatched to survive within. More on this below.

It’s also important to note that such blue light luminaires will not achieve complete disinfection by themselves (though that isn’t necessarily the desired outcome). The research to date has also generally consisted of controlled laboratory experiments and clinical applications. I have yet to come across any research specifically looking at applications in other environments beyond clinical settings.

This would suggest that careful consideration of all of these variables is needed when deciding whether or not, or how to use, such luminaires, and it should include consultation with relevant specialists beyond lighting designers, manufacturers or electrical engineers. But even if such specialists are consulted, the relevant literature makes it clear that more research is needed to obtain a better understanding of how different contextual, or proximate, interactions of these variables impact the effectiveness of blue light as a disinfectant. One epidemiologist I consulted (William Hanage, personal communication, November 30, 2018) questioned if it makes sense to perform such sterilization in wide ranging settings.

He suggested that the current practice of using portable UV lighting units or similar functioning devices to disinfect key rooms or specific surfaces in the absence of people may be the better option. While these spaces or surfaces would become contaminated the next day as soon as they were used again, it would still minimize the proliferation of pathogenic bacteria and the potential for human infection. However, limited applications of blue light luminaires in clinical applications do offer the promise of helping prolong the lifespan of existing antibiotics. And other more targeted applications also have potential, including agriculture and food production (Gwynne and Gallagher 2018).

Question 1: What is the potential for the development of blue light resistant bacteria, particularly if such luminaires were to become widespread – essentially disinfecting the majority of the built environments they’re installed in for large portions of the day (as opposed to limited use in more targeted applications like healthcare)?

The development of antibiotic resistance is a population-level phenomenon that occurs via evolutionary forces, such as natural selection. Inevitably, some bacteria will develop a random mutation providing resistance to a given antibiotic, increasing its ability to adapt and thrive in this new environment (E2) with antibiotics. They survive and pass on their mutation through vertical or horizontal transmission to other bacteria. Resistance to specific wavelengths could develop in a similar manner. Theoretically, over time, bacteria could develop a random mutation that provides more resistant to these wavelengths, which they then pass on. The concern is that the more widespread this technology usage becomes, the greater the chance such a mutation could develop and spread through bacterial populations.

There seems to be general consensus that the way blue light impacts bacteria decreases the likelihood of developing resistance or tolerance to blue light compared to antibiotics. Briefly, the function of resistance refers to a series of evolved genetic mechanisms (history dependent) that allow bacteria to directly resist antibiotics (or other germicides) while the bacteria develop – growing and reproducing (Yong, 2017). However, the function of tolerance refers to a specific “behavioral” strategy of bacteria (also with an underlying historical genetic component) to go dormant, or grow very slowly, while the bactericidal agent is present, essentially waiting it out. And studies suggest tolerance may lead to resistance in some cases.

Antibiotics target a single function within a cell, usually a component of either DNA synthesis, cell wall synthesis, or protein synthesis. In response to this, bacteria have four general mechanisms available for generating resistance: antibiotic inactivation, target modification, altered permeability, or the bypass of that part of the metabolic pathway (Kapoor et al. 2017). Essentially bacteria have four general arrows in their collective quiver to generate resistance against antibiotics’ single targeting means within the cell, increasing the effectiveness for generating resistance to antibiotics.

However, blue light kills bacteria through a reaction with the cell that produces a reactive oxygen species (ROS) (Cabiscol et al. 2000). These ROS damage the bacteria by targeting multiple cell functions simultaneously, including DNA, RNA, proteins, and lipids. With multiple simultaneous targeting methods to address, the four general arrows of bacteria are less effective for developing resistance. It’s harder to produce mutations resistant to blue light (either through a variation of one of the existing arrows or developing a new arrow) that wouldn’t also kill the bacteria itself (or make it less fit with regards to some other aspect of bacteria survival and reproduction).

For tolerance, research (Tomb et al. 2017) has shown that the ROS produced via blue light exposure also makes it more difficult for tolerance to develop, at least for the types of bacteria studied. And the deployment of blue light in ambient lighting systems mean that tolerant bacteria would have to “lay low” for longer periods of time, potentially further increasing the difficulty for tolerance to evolve.

But while there is general agreement in the increased difficulty of bacteria developing resistance or tolerance to blue light compared to antibiotics, there is less agreement as to whether this means it couldn’t happen under some real-world conditions. On the one hand, many studies (Giuliani et al. 2009; Gwynne and Gallagher 2018; Maclean et al. 2014; Tavares et al. 2010; Tomb et al. 2017; Zhang et al. 2014) looking at the development of blue light resistance or tolerance in laboratory settings using culture-based studies have found little evidence supporting the development of either.

On the other hand, some laboratory studies (Guffey et al. 2013; Gwynne and Gallagher 2018; Zhang et al. 2014) have found supporting evidence for this. Some researchers conclude that the exposure of bacteria to lower-intensity, sub lethal stress levels of blue light could lead to the development of tolerance, particularly when the environment is otherwise favorable for bacteria growth and reproduction. Though not all researchers agree with this interpretation.

It’s important to note that all of these studies take place in laboratory settings, exposing limited types of bacteria in isolation from other microbes to specific blue light wavelengths at specific energy levels for limited amounts of time. This is usually repeated for 10 to 20 cycles in the studies reviewed and referenced throughout (collecting surviving cells, re-growing to higher quantities and then re-exposing to blue light). While this typically represents a significant number of generations for most bacteria, and therefore opportunity for resistance or tolerance mutations to develop, it still pales in comparison to the potential number of generations that bacteria could be exposed to blue light in actual settings (consisting of dynamic, microbial communities). Much greater time frames (combined with widespread use) would likely provide greater opportunity for tolerance or resistance to develop through stages of mutations as opposed to a single mutation (e.g. Losos 2017:219-308).

As with the general effectiveness of blue light as a disinfectant, the potential for the development of resistance or tolerance is impacted by a variety of factors, including the type of bacteria, specific wavelengths, energy levels, time of exposure, and other environmental factors like the amount of “food” available for bacteria (Wang et al. 2017) that these studies haven’t fully explored. Gilbert and Stephens (2018) also point out that the influence of the built environment’s real-world conditions on microbial stress, potentially leading to tolerance, resistance, or other negative human health impacts, is an area of research that is still largely unexplored.

It seems that future research should look more specifically at the actual conditions occurring in the various environments these luminaires are being proposed for use in (including the time spans involved), how adaptive the resistance/tolerance mechanisms of various bacteria types are to these conditions, the potential for evolving resistance/tolerance mechanisms over longer periods of times in these conditions, and what the potential negative short and long-term impacts are to human health. Does a new environment (E2) of bacteria more resistant or tolerant to blue light create an evolutionary mismatch for humans relative to our ability to live with or resist such bacteria?

To summarize:

  • While the chances of resistance/tolerance to blue light developing appear to be significantly lower than for antibiotics, the proposed widespread use of this technology (compared to the more constrained aspects of these studies) will likely increase the opportunity for blue light resistance/tolerance to develop.
  • Studies of blue light resistance/tolerance haven’t considered this type of widespread use in less clean or controlled environments (to my knowledge). Such environments, like school classrooms, could prove to be more “nourishing” for bacteria compared to healthcare settings.
  • Real world conditions will likely have blue light wavelengths and energy levels that vary from what was used in these studies. There will likely be significant opportunities for bacteria to be exposed to lower-intensity, sub lethal stress levels of blue light (potentially leading to tolerance), depending on specific system configurations like fixtures layouts and dimming capabilities.
  • Only certain bacteria types have been studied up to now, and only in isolation, which isn’t reflective of the dynamic communities of microbes that live in the multiple types of environments these luminaires are being marketed to. Therefore, we have a limited understanding of the potential dynamic configurations of resistance and tolerance mechanisms that such luminaires will encounter, as well as their development over the life of the individual microbes and their collectives (e.g. Coil and Eisen 2019)
  • These studies generally conclude additional research is needed to further understand the potential development of blue light tolerance and resistance in different types of bacteria, and how different environmental variables impact this.

In addition, the epidemiologist I consulted (William Hanage, personal communication, November 30, 2018), while admitting to not being an expert on the impacts of blue light on bacteria, did think the chances of blue light resistance happening quite high if the technology was widely used. So, while research to date indicates blue light tolerance or resistance is difficult to develop (at least under certain laboratory conditions), it nevertheless appears additional research is needed to fully answer this question if the technology were widely used. However, I should point out that some researchers and manufacturers have concluded the chances are small enough to be of little concern, particularly considering the growing challenges we face relative to antibiotic resistance, infection control, and food contamination.

Question 2: Could the widespread implementation of such luminaires have a negative impact on the populations of bacteria in the larger built environment, whether they’re pathogenic, non-pathogenic or beneficial bacteria?

For bacteria that live within human bodies, such as the beneficial microbiome in the human gastrointestinal system, ambient blue light technology poses no threat that we know of since the wavelengths only penetrate the first few layers of the epidermis (Vital Vio, Inc. 2019). For bacteria (pathogenic or non-pathogenic) that live on the skin, within the first few layers of skin, within glands, or hair follicles, they will be impacted just like other bacteria living on any surfaces exposed to the blue light, potentially even creating a new environment (E2) by shifting populations to a greater composition of bacteria types less sensitive to blue light (Martin Hessling, personal communication, February 25, 2019).

However, since only a small percentage of skin will typically be exposed to blue light at any given time (due to clothing), the potential for creating a negative mismatch relative to the human immune system or other aspects of human health is likely small. Microbes living on the face and hands (or other exposed areas), if impacted, are generally repopulated from covered portions of the body, or from the surrounding environment. This is similar to what happens when we shower or wash our hands (Vital Vio, Inc. 2019). In other words, the E2 environment may not be that dissimilar from the E1 environment with respect to impacts on human health.

The epidemiologist (William Hanage, personal communication, November 30, 2018) consulted basically thinks the likelihood is low for negatively impacting non-pathogenic or beneficial bacteria on or in humans. And, in fact, he speculates that blue light could help prevent the very low likelihood that any of the “good” bacteria that lives within us, if after ending up on a surface, would live long enough to mutate into a virulent form and spread, creating a new environment (E2) less favorable to the human health

However, there are still the unanswered questions of a) the potential negative impact on populations of pathogenic, non-pathogenic, or beneficial bacteria in the larger built environment (e.g. Gilbert and Stephens 2018), and b) what the potential ramifications of those impacts could be. At least I haven’t come across such studies (specifically relative to blue light in non-clinical applications), though admittedly such studies outside of the laboratory can be more difficult to do depending on a study’s focus (Coil and Eisen 2019); however, while culture-based studies have dominated throughout the 2000s, that now appears to be changing (Gilbert and Stephens 2018). Regardless, the varying, dynamic communities of microbes that exist within our built environments means there are a lot of unknowns for interactions with blue light technology, and the potential mismatches for human health that might occur as a result.

For example, setting aside the potential to develop resistant or tolerant bacteria through mutations, what might happen simply as a result of shifting the composition of bacteria populations in the wider built environment to species or strains currently less sensitive to blue light? Could this new environment (E2) relative to bacteria composition in and of itself lead to problematic tolerance or resistance? How might this disrupt microbial communities and could that lead to a negative mismatch for the human immune system or other aspects of human physiology? How do other environmental factors, ranging from the specific composition of bacteria populations (with unique histories, mechanisms, and development patterns) to indoor environmental quality conditions like temperature and relative humidity or the chemical composition of finish materials also impact this (e.g. Gilbert and Stephens 2018)? Our understanding of the built environment’s microbiome is still at a basic level.

Just as there has been concern that the overuse of antimicrobials (including antimicrobial surfaces) could give rise to resistant bacteria or disrupt microbial communities in the larger environment (Grenni et al. 2018), it would seem these germicidal lighting questions should be adequately answered before being widely used commercially. Again, though, some researchers and manufacturers see this risk as small.

Question 3: Are there potential direct damaging impacts to humans from long term exposure to wavelengths in the 405nm range at these intensity levels?

In response to this and similar questions, some manufacturers of this technology have pointed out that several studies (Bache et al. 2012; Dai et al. 2012b; McDonald et al. 2013; Ramakrishnan et al. 2014; and Zhang et al. 2014) have exposed mammalian and human cells to blue light at antimicrobial intensity levels without causing damage to them. However, it’s important to note that the focus for each of these studies was disinfecting applications in clinical settings, under limited time durations, primarily to prevent the development of hospital associated infections. As with the question of resistance or tolerance, these studies do not look at uses providing a greater area of coverage, different types of environments, daily exposure for significant amounts of time/day, and repeated daily exposure over one’s life (i.e., the actual E2s being proposed by manufacturers). And these studies’ authors also generally conclude that more research is needed for even the more limited applications they are focusing on. One study (Daia et al. 2012a) also indicated more research was needed to determine the long-term impacts on mammalian cells.

Manufacturers of this ambient germicidal blue light technology will also point out that they comply with the International Electrotechnical Commission (IEC) 62471, Photobiological Safety of Lamps and Lamp Systems. This is a testing and classification standard assessing the photobiological safety of lamp technologies and other sources of optical radiation (including LED lights) relative to potential impacts on human skin, the front surface of the eye, and the retina (UL 2019). The compliance process consists of “… 1) measuring absolute radiance and irradiance levels, 2) comparing effective (‘weighted’) levels with the exposure limits defined by the standard, and 3) determining a risk group to which a product is assigned based on the level of hazard to the skin and eye (UL 2019).”

There are four risk groups, defined by how long it takes to reach exposure limits. These blue germicidal ambient lights will have been tested against this standard, and if successful, will be assigned to the Exempt risk group (no risk), meaning that it takes 10,000 seconds or longer to reach exposure limits. It is, however, important to be clear exactly what this testing and classification standard doesn’t tell us.

IEC 62471 doesn’t address a) sensitive populations (those with pre-existing eye/skin conditions, children, or elderly populations), b) chronic or repeated exposure (exposure limits are based on continuous exposure up to 8 hours max), and c) long-term exposure (Martinsons 2015). All three of these are relevant to widely marketing this technology for general use spaces in schools, offices, dorm rooms, hospitality, etc., which raises concerns that some populations could be negatively impacted. One researcher in this area contacted directly (Martin Hessling, personal communication, February 25, 2019) agreed that this concern is warranted for such uses relative to these populations.

But even for non-sensitive populations, the potential negative impacts of chronic/repeated exposure and/or long-term exposure haven’t been fully addressed. One such long term consideration is the formation of cataracts, which is promoted by violet radiation (near the 405-410 peaks of germicidal ambient luminaires) (Hessling et al. 2018). However, violet light (compared to “blue” light closer to a 450nm peak) is more likely to be absorbed by the ocular media in front of the retina, especially the lens, before making it to the retina, reducing its relative hazard in the short term (at least for adults). But over the long term, enough may still reach the retina to be problematic, especially for sensitive populations. We simply don’t know yet.

Much of the above discussion focuses on how aspects of our physiologies develop over the course of our lives (determined by our evolutionary history in our ancestral environment (E1)), and how different life stages may present different susceptibilities to blue light. While exposure to blue light during the healthy adult stage may present few health risks (at least for short term exposure), those risks to the eyes and skin may increase for children and the elderly (for different reasons related to the development of human physiological systems over the course of our lives), particularly if experiencing chronic, or long-term exposure. The potential for mismatch in these populations hasn’t been adequately studied.

An additional avenue of exploration would be to compare the blue light hazard risk this technology poses to that of daylight/sunlight, which we evolved under and spent the majority of our evolutionary history experiencing – the ultimate “ancestral” environment (E1) for humans. One study (Hessling et al. 2018) has previously made such a comparison of daylight/sunlight to various existing lighting technologies, though unfortunately germicidal blue ambient luminaires weren’t included in the comparison.

Each of the sources examined (warm and cool white LEDs, iPad Mini 2 in day and night mode, halogen source, and a fluorescent tube) had a relative blue light hazard at or below that of daylight/sunlight, which approximated midday Berlin sunlight in April or September in an uncloudy sky. Without performing the calculations for a specific germicidal blue ambient lighting fixture, we can’t know where exactly it would fall relative to daylight/sunlight, but I wouldn’t be surprised if such luminaires have a slightly higher relative blue light hazard value.

Even if they don’t, the calculated blue light hazard value of daylight/sunlight used in this study is an annual, daily, and sky condition maximum (though not a maximum for geographic location, which would likely increase as you moved closer to the equator). At other times of the day and year, under partly cloudy or overcast conditions, or at other geographic locations farther north, the relative blue light hazard for daylight/sunlight will be less than what’s calculated in this study, and less than that of other light sources, including the germicidal blue ambient luminaires (as indicated by the study’s authors). The point being that interior conditions (the “new” (E2) environment) could still result in relative blue light hazard conditions greater than what’s outside (the “ancestral” (E1) environment) at any given time (particularly after sunset).

While research to date suggest the short-term hazards (evolutionary mismatches) of blue light to human cells are minimal for some populations (i.e., healthy adults), we still don’t have a good grasp on the potential hazards presented to sensitive populations. Nor does it seem we have a grasp of the potential impacts from chronic/repeated exposure or long-term exposure. But again, some researchers and manufacturers would argue that the risk to non-sensitive populations is minimal even for chronic/repeated or long-term exposure. I would argue the question hasn’t been answered yet.

Question 4: Could the widespread implementation of such luminaires have a negative impact on the development and maintenance of the human immune system? Particularly considering that these luminaires are being marketed to schools.

To date I’ve found comparatively less information on this question. The experts and manufacturers consulted, and literature reviewed, all indicated more research is needed to provide an answer. From the epidemiologist’s perspective (William Hanage, personal communication, November 30, 2018), this is a complex question that depends in part on the contextual situation (including how widespread the use of this technology is), the demographic groups involved, and the specific immunities being focused on. Similar to the discussion of the potential impacts on human eyes and skin, there is also the question of how the development of the immune system over one’s life could be impacted. Maybe the risk for a mismatch is greater at certain stages of our life compared to others.

The “hygiene hypothesis,” supported by some studies, suggests that improved hygiene, or hygiene overdone, in our modern built environments (of which widespread use of blue light luminaires could be considered an example of) may be a contributing factor in the rise of autoimmune disorders (Gilbert and Stephens 2018), though this hasn’t been sufficiently studied. Conceivably this could be a greater issue for children than healthy adults.

Another researcher consulted (Martin Hessling, personal communication, February 22, 2019) hypothesized that the shifting to less sensitive bacteria populations that may occur under such luminaires could change the environmental constraints that shape immune system development. The hypothetical example he provided was the replacement of Staphylococci (less resistant to blue light) by E. coli or other bacteria more resistant to blue light in environments illuminated by this technology. Ultimately, he thinks more research is needed. Gilbert and Stephens (2018) would agree, stating that we don’t know yet what the right balance is between too much and not enough exposure to microbes, whether we’re talking about the development and maintenance of the human immune system or human health in general. Conducting studies that systematically compare the microbial conditions between our “ancestral” environment and the “new” built environments using blue light luminaires seems like a good place to start.

Question 5: What are the potential impacts to the human circadian system from consistent exposure to wavelengths in the 405nm range throughout the day and into the evening?

A full discussion of the mechanisms of our circadian system and how lighting impacts those mechanisms is beyond the scope of this article. Safe to say, the lighting community is not in full agreement of every detail of the human circadian system, what the best metrics are to use, or how to design for optimal circadian system operation. For the purposes of this discussion, I am adopting the circadian system perspective of the Lighting Research Center (LRC) at the Rensselaer Polytechnic Institute.

In order to help designers and specifiers account for circadian rhythm needs in the built environment, the Lighting Research Center has developed various metrics and tools, including the circadian light (CLA) and circadian stimulus (CS) metrics. They are based on our understanding of retinal physiology and how the retina converts light into the neural signals used by our circadian system. This conversion process is known as circadian phototransduction (Figueiro and Rea 2017) and results from human evolutionary history in our ancestral (E1), exterior environment.

Basically, CLA is the irradiance on the eye weighted by the spectral sensitivity of all the relevant phototransduction mechanisms that are stimulating the biological clock to suppress melatonin production. The CS metric then transforms CLA into relative units, ranging from zero (the threshold for activating the circadian system) to 0.7 (response saturation). This relative value is directly proportional to the suppression of melatonin after one hour of light exposure (Figueiro and Rea 2017).

Research has shown that a CS of 0.3 or greater at the eye, for at least an hour in the early morning, is enough to generate a circadian system response and drive down melatonin production. General recommendations are to provide a CS of 0.3 during the morning through early afternoon and then reduce to less than 0.1 by late afternoon and into the evening so that melatonin production can ramp back up for sleep (Acosta et al. 2015; Figueiro and Rea 2017; Figueiro et al. 2016; Lighting Research Center 2012a).

The Lighting Research Center has also developed a CS calculation tool (either excel or online versions) available for use by designers to help select the best light sources and light levels to achieve the right circadian light exposure at the right time of day. Using manufacturer supplied light source spectra, including spectra for germicidal ambient blue light sources, designers can calculate CS and CLA values. I have yet to obtain spectra for a germicidal ambient light source, but one manufacturer indicated the white visible light disinfection spectrum used in their luminaires has a CS value of 0.121, assuming a pupil illuminance of 100 lux, which is in the middle of the Illuminating Engineering Society’s (IES) recommended range for vertical illuminance values in an office environment. With increasing or decreasing pupil illuminance, the associated CS value would respectively increase or decrease from that.

Further calculations would be required to verify, but this suggests that through dimming of these blue light luminaires (potentially combined with changes to color temperature or other LED variables) the CS value could be lowered enough to meet the late afternoon/evening recommended CS values. However, illuminance levels low enough to meet this recommendation may not be enough to meet other visual task or space related needs. Nor may the lower levels be intense enough to provide the intended germicidal function. This would need to be verified with the manufacturer after the CS calculations had been performed.

Whether or not a luminaire’s full intensity is enough to meet the morning recommended CS values would also have to be verified through calculations. If the recommended values couldn’t be met, the desired morning circadian system response would need to be generated through other means. It’s also possible that whatever intensity values are needed to reach a CS of 0.3 (if provided by the blue light luminaires) could have negative blue light hazard implications, particularly relative to the unknowns of chronic or repeated exposure. This would also need to be verified after the calculations were run.

So, there is potential for germicidal ambient blue luminaires to be used in manner that doesn’t have a negative impact on our circadian systems – that doesn’t create a circadian system mismatch. Though it will depend on specific fixture spectra, the use of dimmable systems, specific occupant tasks, specific space configurations, and potential blue light hazards from use at higher intensities (if attempting to generate a morning circadian system response). CS calculations would be needed on a project by project basis to verify. And avoiding such mismatches is important because they can result in a large variety of negative health impacts throughout the development of various human physiological systems (Harmon 2018b).

I did reach out to the Lighting Research Center to get their take on this. They agreed that performing CS calculations would be an appropriate step for assessment of specific manufacturer luminaires in a given context (Mariana Figueiro, personal communication, December 2, 2018). However, the center was unable to share much more information. They are currently involved in a research project evaluating germicidal/disinfecting ambient lighting fixtures, but because they’re under an NDA relative to this research, they were unable to discuss any of the details. This suggests the LRC believes the potential impacts on the human circadian system is an issue requiring further study.


Based on the above discussion, the R&D team of BranchPattern is currently recommending to our engineers and designers that they tread carefully when considering specifying germicidal ambient luminaires. There are still unanswered questions relative to the impacts on a) bacteria resistance and tolerance, b) microbial communities in the larger built environment, c) sensitive human populations, d) human cells from chronic/repeated exposure, e) human cells from long-term exposure, f) the human immune system, and g) the human circadian system.

Nevertheless, certain hospital, veterinarian, cleanroom, agricultural, food production, restroom, gym, and transportation settings may still be good candidates for this technology once the importance of the above unanswered questions have been weighed for the specific project being considered. More caution is warranted for settings like school classrooms, daycares, hospitality, offices, and dormitory rooms, at least for now. But there are likely specific spaces within these settings, such as food prep areas or restrooms, that may still be appropriate. A night mode use may be appropriate for some of these settings as well.

In every case, relevant experts, such as medical professionals, microbiologists, epidemiologists, the manufacturers of the technology, etc., should be consulted to help determine how best to use them in the specific proximate contexts being considered, or whether to use them at all instead of other options like portable UV lighting units. And such efforts, as well as future research to answer remaining questions, should be completed within an evolutionary framework that a) distinguishes between ultimate and proximate causation, b) accounts for Timbergen’s four characteristics of traits, and c) recognizes the potential for evolutionary mismatches. Otherwise we risk creating “new” environments misaligned with a given occupant population’s short- and long-term needs – with their physiological mechanisms and developmental patterns that evolved in a different “ancestral” environment. It’s possible we’re being overcautious, and some researchers and manufacturers would argue we are. But to make sure we’re improving life, we think it’s prudent the above questions have better answers before moving beyond these recommendations.

Read the full Evolutionary Mismatch series:

  1. Introduction: Evolutionary Mismatch and What To Do About It by David Sloan Wilson
  2. Functional Frivolity: The Evolution and Development of the Human Brain Through Play by Aaron Blaisdell
  3. A Mother’s Mismatch: Why Cancer Has Deep Evolutionary Roots by Amy M. Boddy
  4. It’s Time To See the Light (Another Example of Evolutionary Mismatch) by Dan Pardi
  5. Generating Testable Hypotheses of Evolutionary Mismatch by Sudhindra Rao
  6. (Mis-) Communication in Medicine: A Preventive Way for Doctors to Preserve Effective Communication in Technologically-Evolved Healthcare Environments by Brent C. Pottenger
  7. The Darwinian Causes of Mental Illness by Eirik Garnas
  8. Is Cancer a Disease of Civilization? by Athena Aktipis
  9. The Potential Evolutionary Mismatches of Germicidal Ambient Lighting by Marcel Harmon
  10. Do We Sleep Better Than Our Ancestors? How Natural Selection and Modern Life Have Shaped Human Sleep by Charles Nunn and David Samson
  11. The Future of the Ancestral Health Movement by Hamilton M. Stapell
  12. Humans: Smart Enough to Create Processed Foods, Daft Enough to Eat Them by Ian Spreadbury
  13. Mismatch Between Our Biologically Evolved Educative Instincts and Culturally Evolved Schools by Peter Gray
  14. How to Eliminate Going to the Dentist by John Sorrentino
  15. Public Health and Evolutionary Mismatch: The Tragedy of Unnecessary Suffering and Death by George Diggs
  16. Is Shame a Bug or a Feature? An Applied Evolutionary Approach by Nando Pelusi
  17. The “Benefits,” Risks, and Costs of Routine Infant Circumcision by Stephanie Welch
  18. An Evolutionary Perspective on the Real Problem with Increased Screen Time by Glenn Geher
  19. Did Paleolithic People Suffer From Kidney Disease? by Lynda Frassetto
  20. The Physical Activity Mismatch: Can Evolutionary Perspectives Inform Exercise Recommendations? by James Steele


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Published On: April 4, 2019

Marcel J. Harmon

Marcel J. Harmon

Marcel J. Harmon, a licensed professional engineer and anthropologist, received his Ph.D. in Anthropology from the University of New Mexico. He currently co-leads the Research & Development team at BranchPattern, a building consultancy dedicated to improving life through better built environments. The primary mission of the team is to provide a research/evidence-based approach for aligning design intent with occupant and organizational needs. Over the years Marcel’s academic and professional focus have included applications of evolutionary theory to understanding past and contemporary societies and the reciprocal relationships between people and their built environments. In his current role, Marcel leads research projects designed to provide insights relative to specific client questions. He engages building occupants, gathering their stories and personal narratives, to ensure that projects better account for occupant’s wants and needs. He also quantifies the built environment’s impact on occupant productivity/performance and health, as well as the occupant’s impact on building performance. Marcel uses this understanding to inform on the process from early programming through post occupancy evaluations, and encourage longer term, prosocial decision making during the design/construction process.


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