Indoor Air Quality: Out of the Shadows, But Still on the Back Burner

Since the beginning of the modern environmental movement in the 1970s, air pollution has been defined as the debasement of the outdoor ambient air. That movement has been a resounding success. Accompanying these benefits has been the recognition that the indoor environment is now relatively more important. The EPA has responded with citizen advisory programs but little information on indoor risk or remediations.

The National Academy of Sciences (NAS) recently released a Consensus Study Report, “Health Risks of Indoor Exposure to Fine Particulate Matter and Practical Mitigation Solutions,” similar to the Criteria Reports issued by EPA concerning outdoor ambient air pollutants. THE EPA requested the NAS report and provided guidelines. The project's stated purpose was to “consider the state of the science on the health risks of exposure to fine particulate matter indoors…”

The COVID-19 pandemic taught many hard lessons. Among the most prominent of these was the tight connection between the indoor environment and health.

- NAS Report

Indoor Pollutants

This objective is not precise; it could mean indoor exposure to outdoor pollutants that have infiltrated or indoor exposure to pollutants that were emitted indoors; their compositions differ substantially. Outdoor pollution is typically rich in inorganic ions (sulfates, nitrates, ammonium), metals, crustal materials, and reactive carbon compounds. Particles emitted indoors may include soot, pet dander, cigarette smoke, aeroallergens, household dust, cleaning products, and chips of lead-based paints.

Sources of these emissions are likewise essential. Outdoor sources, such as power plants, highways, agriculture, or processing plants, tend to be large and specific, with effluents widely dispersed during atmospheric transport. Concentrations of the resulting mixtures have been controlled by setting regulatory emission limits on individual sources. I refer to these situations as “wholesale.” It is virtually impossible to assign blame for an urban dirty air day to any specific emission sources; the responsibility is collective to the area or region.

Pollution generated indoors is an entirely different matter. Indoor sources are discrete, i.e., “retail” and, most often, the responsibilities of occupants. Abatement is simple: you bought it, you own it, and you can turn it off. We may rely on environmental or public health agencies to describe risks incurred by specific indoor sources, i.e., candle flame, air freshener, and pet dander; however, controlling them lies with us. Concerns over inhaling second-hand cigarette smoke are a case in point.

Infiltration of outdoor air complicates our personal control strategies. Fifteen air changes per hour are recommended for human comfort concerning humidity or CO2 levels. Higher exchange rates will increase concentrations of outside air and vice versa. Residents need to know which is riskier: outdoors or indoors. Such choices will vary by season and time of day.

NAS Conclusions

Although the NAS report is a welcome contribution to the technical literature, it falls short in three key aspects:

  • Absence of dose-response information and discussions of indoor vs. outdoor epidemiology.
  • Focusing on particulate matter to the exclusion of other pollutants such as nitrogen oxides or organic pollutants. 
  • Focusing on a specific particle size range (2.5 µm), neglecting the effects of ultrafine (UFP) and larger particles (PM10). [1]
  • Neglecting common indoor pollutants

EPA’s ongoing fixation on PM2.5 stems from epidemiology based solely on outdoor air quality, in which indoor contributions were neglected, and total exposures were thus underestimated. The correct approach would have been to characterize all indoor pollutants and re-evaluate the extant epidemiology. The current situation results in underestimating exposures, overestimating health effects, and masking thresholds.

I have a suggestion for follow-on research. Each of the 11 NAS contributors should provide an indoor PM2.5 sample from each room in their residence with and without air conditioning, thus creating a dataset to be sent to the appropriate NIH labs for spectrographic, chemical, and particle size distribution analysis. This should be adequate to demonstrate diversity in residential air quality in relation to typical outdoor air samples. The current situation concerning indoor air quality resembles that of outdoor air quality in the 1970s. It is now time to collect data.


[1] As a reminder, PMxx characterizes species by particle size rather than composition.

Source: Health Risks of Indoor Exposures to Fine Particulate Matter and Practical Mitigation Solutions National Academy of Science, Engineering, and Medicine (NAS)