The Need For US Indoor Air Quality Guidelines

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Advancing Policy for Healthy Buildings at the Federal and State Level

Executive Summary

Indoor Air Quality Guidelines Inform Climate and Health Solutions

Climate change and human health are both tightly linked to fossil fuel use in energy systems, including those within residential and commercial buildings. Greenhouse gas emissions will increase the frequency and severity of extreme climate events over the next century, while exacerbating current health problems and introducing new ones. In the near-term, indoor and outdoor air pollution from combustion contributes to poor health outcomes, including respiratory and cardiovascular disease.

However, the importance of indoor air quality is often overlooked. Decarbonizing the buildings in which we live, work, and play is a key solution to both limit climate impacts and promote human health and safety by reducing combustion pollution exposure. Adopting indoor air quality guidelines in the US would improve our understanding of indoor environments and inform health-protective strategies in the building sector, including electrification and beyond.

Understanding and Protecting Indoor Air

Poor indoor air quality is one of the top five major public health risks, yet “healthy” indoor air quality is not well defined, and indoor air is largely unregulated by federal and most state governments.

Setting indoor air quality (IAQ) guidelines provides a foundation for understanding and addressing indoor health risks. Guidelines can inform standards, regulations, and policies to ensure that indoor environments are safe and healthy. IAQ guidelines are not new or novel. More than 50 organizations — including the World Health Organization (WHO) — across 38 countries have established IAQ guidelines. In the United States, the Environmental Protection Agency (EPA) has already set an indoor guideline for radon, and states including California, Illinois, and Texas have established some form of IAQ guidelines.

Federal and state agencies can and should act to protect families from unhealthy indoor air by setting new IAQ guidelines, using existing IAQ guidelines as models, and increasing research to inform and update guidelines. In this report, RMI provides a comprehensive, novel review of IAQ guidelines, summarizes their real-world applications, and provides recommended actions for policymakers.


The Evidence Is Clear: Indoor Air Has an Impact on Health

Exposure to indoor air pollutants poses a serious health threat. We spend almost 90% of our time indoors, where pollutant concentrations may be two to five times higher than outdoors. Behavioral shifts since the COVID-19 pandemic have further increased time spent indoors, and many of those behavioral changes have been sustained. Changes to outdoor environmental conditions due to climate change are projected to cause additional adverse effects to indoor air quality that will harm human health, including increased indoor heat stress, mold, ozone exposure, and wildfire smoke exposure.

Exhibit 1: Key Indoor Air Pollutants

Pollutants were selected based on their inclusion in published guidelines, established scientific evidence of human health effects from exposure, and their known presence in indoor environments. This list of pollutants and descriptions are non-exhaustive.

A growing body of research links poor IAQ with adverse health outcomes. Exposure to indoor air pollution can contribute to respiratory issues, hospital visits, absences from school and work, and serious conditions such as heart disease, stroke, and cancer.

Risks may be more severe for vulnerable populations including children, the elderly, and people with existing respiratory issues, who are likely to spend more time indoors. Low-socioeconomic households experience a disproportionately high risk of pollutant exposure indoors, where housing conditions may be smaller, more crowded, older, and closer to outdoor pollutant sources. Health risks may also be higher for homes that use gas stoves, where nitrogen dioxide (NO2) levels can be around 50 to 400 times higher than in homes with electric stoves. More guidance about healthy IAQ is needed to better inform building occupants about IAQ risks, especially for vulnerable groups who already face increased risk of health impacts from climate change.

Indoor Air and Outdoor Air: Similar but Different

The Clean Air Act (CAA), EPA’s ambient air regulation, has sometimes been used to assess IAQ. Ambient air is defined in the CAA as outdoor air: “air external to buildings.” Although ambient pollutants do travel indoors, the indoor environment is distinct, and it is therefore not precise to apply ambient air guidance.

Indoor air contains disparate and often higher concentrations of pollutants than ambient air. Some indoor air pollutant concentrations can also be greatly influenced by seasonal variability, and as previously noted, people spend most of their time indoors.

Because of these factors, indoor environments have the largest influence on personal pollutant exposure, and ambient measurements of pollutants may underestimate long-term health risks associated with exposure. Indoor-specific guidelines for key pollutants (see Exhibit 1) are necessary to accurately understand health risks.

Although not applicable indoors, the CAA’s regulatory framework shows the potential of US policy to protect air quality and human health. Under CAA enforcement, collective emissions of the six most common ambient air pollutants decreased 78% from 1970 to 2020. This drastic reduction drives lower mortality rates as well as fewer respiratory illness cases, emergency visits, and absences from school and work.

Benefits of CAA pollutant control are estimated to exceed costs by more than 30-to-1, resulting in over $2 trillion in estimated avoided health costs in 2020. Parallel protections for indoor air could result in additional reductions in illnesses, deaths, hospital admissions, and absences.

By setting IAQ guidelines, federal and state agencies can help reduce harmful pollutants indoors and associated health risks and costs.

What Are Indoor Air Quality Guidelines?

Generally, and as defined in this report, IAQ guidelines are science-based, voluntary limits on indoor air pollutant concentrations. As opposed to standards, guidelines are not mandatory or enforced. Rather, guidelines give health- or risk-based benchmarks on safe pollutant thresholds. Guidelines may be developed using various methods:

  • Some guidelines set health-based pollutant concentration limits that eliminate all health risks or minimize risks as much as possible.
    • WHO 2010 guidelines identify a pollutant exposure threshold (“reference concentration”) at which health risks are eliminated or minimized. WHO 2021 guidelines similarly identify the lowest level of exposure for which there is evidence of adverse health effects.
  • Other guidelines set feasibility-based pollutant concentration limits that minimize major health risks and set achievable exposure limits.
    • Health Canada’s guidelines, such as the 2021 guideline for carbon dioxide (CO2), identify a reference concentration below which serious health problems are unlikely to occur. A feasibility analysis is then conducted to ensure that the reference concentration can be achieved through controlling indoor pollutant sources and other mitigations. If pollutant levels cannot be lowered to the reference concentration value, the reference concentration may be set at a higher, more achievable value.
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