Chemical versus biological contamination indoors: trade-offs versus win–win opportunities for improving indoor air quality


  • Nicola Carslaw,

  • Abigail Hathway,

  • Louise Fletcher,

  • Jacqueline Hamilton,

  • Trevor Ingham,

  • Catherine Noakes

We recently held a workshop that brought together, perhaps for the first time, researchers in the fields of atmospheric and indoor air chemistry, infection control engineering, building physics, and environmental microbiology. As separate groups of scientists, we tend to focus on discipline-specific problems. Indoor air chemists are largely concerned with chemical reactions indoors and the resulting products and then trying to identify those that may be harmful to health. Members of the indoor biological community, on the other hand, are interested in airborne microorganisms and often work at the frontline of pathogen control. For instance, they may be trying to eliminate infectious agents from a hospital environment, where compromised immunities make some occupants much more susceptible to infection.

Both communities advocate for good ventilation. The expanding desire to make buildings more energy efficient, combined with greater knowledge of the composition of our indoor air, has, in recent years, led to numerous technical solutions and devices to control indoor air quality. However, many of these devices create a conflict; a solution that inactivates biological pathogens may inadvertently produce indoor air chemicals that could be a greater health risk for some of those exposed to them.

The hospital environment is a good example of where there is an obvious need to prevent the spread of infection. Controlling ventilation is crucial, with high ventilation rates being recommended in many areas. For instance, in the UK, a general ward in a hospital is recommended to have an air exchange rate of 6/h (Beggs et al., 2008), while an operating theater requires 25/h (Department of Health, 2007). Such air exchange rates are typically 2–30 times higher than those in office buildings and houses. In addition, pressure relationships are important; infectious disease isolation wards need to be kept under negative pressure to prevent air leakage into the general hospital area, while operating theaters are maintained at positive pressure to keep out external contaminants.

Some manufacturers have claimed that one way of minimizing infection risk in locations such as hospital wards is through the use of what we broadly label as ‘air cleaning devices.’ Such devices are also marketed to the commercial buildings sector as an energy-efficient approach to improving indoor air quality, particularly in buildings with recirculating air systems. One only has to conduct a quick search on the Internet to realize that there are a plethora of these products available, claiming a wide range of benefits. The technology behind such devices ranges from simple fibrous filtration that physically removes airborne pathogens to the use of physical or chemical technologies to ‘treat’ the air, including ionization, UV-C irradiation, or the addition of ozone or other chemicals. Some devices disinfect the air as it passes through them; for example, by drawing contaminated room air through a unit containing germicidal UV-C lamps. Other devices emit a substance into a room to react with microorganisms; for example, several devices on the market generate hydroxyl (OH˙) radicals, which are then supposed to inactivate microbial agents and (potentially) chemical pollutants, too.

Some IAQ control devices have data to support their ability to reduce airborne microorganism concentrations, and there is good evidence that some technologies are biologically effective. Germicidal UV-C irradiation is recognized as a mainstream technology for its biological inactivation abilities and is recommended by CDC in the control of tuberculosis (Jensen et al., 2005). Hydrogen peroxide is used commercially in decontamination. While some manufacturers acknowledge health risks associated with their technologies and promote appropriate designs or devices for use in unoccupied spaces, many are designed for occupied rooms with little consideration of the air chemistry involved, and the health risks that secondary pollutants may pose to occupants.

The air cleaning devices that have already been identified as a concern to indoor air chemists are those that produce high concentrations of ozone. The US EPA notes that they are not always safe and effective at removing pollutants and that ozone is itself a lung irritant (US EPA, 2009). The same report also notes that many air cleaning devices are ineffective or not as good as they claim to be. The California Environmental Protection Agency Air Resources Board (2013) have gone one step further and recently introduced regulations that limit the amount of ozone that air cleaners marketed for indoor use in California can emit. However, the risks of high ozone concentrations are relatively well recognized, and few ozone-generating devices are advocated for use in occupied rooms. Air treatment devices that are of perhaps greater concern now are the hydroxyl radical–generating units that are marketed for use in occupied spaces. One instrument we know of contains a reservoir of limonene to ‘kick-start’ the chemistry that aims to inactivate airborne biological material. Ozone–limonene reactions are well known to produce a wide range of short-lived (including OH radicals) and longer-lived pollutants (e.g., formaldehyde, secondary organic aerosol, and nitrated compounds), many of which have known or suspected adverse health effects (Carslaw et al., 2012).

Should we always avoid using such devices, or do they have benefits under certain situations? How do we judge whether it is better to be exposed to the original biological pollutants or the chemical pollutants that are formed when we try to remove them? The answers may depend partly on one's point of view, of course. For patients who have undergone surgery and need critically to avoid infection, taking a chemical ‘hit’ for a few days to avoid exposure to biological pathogens could be an acceptably small price to pay. But how about the asthmatic doctor or a nurse with respiratory problems? How do we decide what is best for them? Their exposures in healthcare facilities occur over a much longer period, and consequently, chemical exposures may have a much greater long-term impact on their health.

We have seen such trade-offs before with environmental indoor air quality. Indeed, a major impetus for the whole research field emerged owing to the unanticipated consequences of energy efficiency measures in the 1970s, which led to complaints such as sick building syndrome. Attaining the proper balance for reducing energy use and cost while maintaining high indoor environmental standards remains a major challenge today.

So are there any win–win situations in the balance between controlling airborne biological and chemical pollutants indoors? What measures could we take, other than maintaining adequate ventilation rates that would lower our exposure to both?

We could think about both the design of these devices and how and where we use them. For example, filtering out ozone downstream of any air cleaning device using activated carbon is a design option where such devices are installed as part of the building ventilation system. For the specific case of hospitals, we could think carefully about which shifts healthcare workers work. We know that exposures to indoor air chemistry products tend to peak in late afternoon with ambient ozone (Apte et al., 2008). For healthcare workers with pre-existing respiratory conditions, avoiding this time of day by adjusting work shifts may be beneficial to their health. Alternatively, adjusting the use of air cleaning devices to only be used during critical procedures during which large amounts of bioaerosols are likely to be emitted into the air could also reduce the total chemical load.

What is very clear is that there is a substantial need for more research to understand the benefits and application of air cleaning devices, and crucially, this research needs to consider both the biological effects and air chemistry in tandem. Moreover, to enable such devices to be used correctly to improve building environmental performance for occupants and contribute to reducing energy consumption, collaboration with building physics and ventilation expertise is also necessary. Our workshop highlighted the benefits of working across disciplines and enabled different research groups to understand the drivers and approaches in each other's fields. Following our workshop, we made cooperative measurements of a range of biological indicators, ventilation characteristics, and short- and long-lived chemical species in two rooms: We will report these measurements in the near future. Such research allows us to start to understand the relationship between airborne chemical and biological contaminants indoors and whether there are suitable ways of reducing the levels of both.