Integrating indoor and outdoor nitrogen dioxide exposures in US homes nationally by ZIP code

Background

Studies of indoor and outdoor air quality have historically been conducted in isolation from one another, limiting our understanding of total residential pollution exposure. Researchers at PSE Healthy Energy are taking a comprehensive look at how indoor and outdoor pollution sources jointly contribute to poor air quality in and around the home.

Outdoor air quality is generally well understood thanks to decades of extensive monitoring and modeling, but the state of indoor air—in contrast—has received less attention until more recent years. Considering people spend nearly 90% of their time indoors, it is critical that we advance our understanding of the sources and effects of indoor air pollution too. One priority pollutant is nitrogen dioxide (NO₂), a respiratory irritant linked to asthma and chronic obstructive pulmonary disease, among other health concerns. Various outdoor sources contribute to NO₂ exposure, including vehicle exhaust, fossil fuel combustion for power generation, and natural gas flaring. Indoors, NO₂ is produced primarily by gas and propane combustion from stoves.

This peer-reviewed study, published in PNAS Nexus, is the first to integrate outdoor NO₂ sources with the primary indoor source—gas and propane stoves—to estimate total residential NO₂ exposures nationwide at the ZIP code level. The study’s lead author, PSE air quality scientist Dr. Yannai Kashtan, PhD, conducted the bulk of this research as a graduate student at Stanford.

The researchers generated the exposure estimates by combining their own measurements of NO₂ emission rates and concentrations indoors with existing data on outdoor NO₂ concentrations, and accounting for a variety of household factors like home size, occupancy, window usage, and ventilation rates. This assessment allowed researchers to evaluate the contribution of gas stoves to residential NO₂ exposures in comparison to all outdoor sources combined.

Key Findings

• Gas and propane stoves are substantial contributors to residential NO₂ exposure. For households that cook with gas or propane, this single appliance is responsible for one quarter of their total average residential NO₂ exposure, whereas outdoor sources collectively account for three quarters.
  • As gas stove usage or intensity climbs, this source accounts for a larger and larger share of total NO₂ exposure. For households in the 95th percentile of gas stove use intensity, representing approximately two hours of stove use per day, gas stoves account for over half (57%) of their total NO₂ exposure across all residential sources.
  • Geography also plays an important role, considering outdoor NO₂ levels trend lower in rural areas and higher in urban areas. For households in rural areas with gas stoves, a larger proportion of their total NO₂ exposure (over one third) is linked to gas stoves. Cumulative NO₂ exposures are higher in urban areas, though, and small apartment-style living spaces contribute to higher NO₂ concentrations indoors.
• When the study’s researchers compared NO₂ exposures to long-term safety thresholds set by the World Health Organization and US Environmental Protection Agency, they identified gas stoves are exposing 22 million Americans to NO₂ levels that exceed these thresholds when their cumulative outdoor exposures otherwise would not.
• Since gas stoves emit NO₂ in bursts, people in homes with gas stoves are regularly exposed to NO₂ concentrations that exceed short-term or “acute” exposure thresholds. The researchers found that virtually all instances of acute residential exceedances are linked to gas stoves rather than outdoor sources.

Conclusion

Although policies have reduced outdoor sources of air pollutants such as NO₂ in the United States, indoor air quality remains largely unmeasured and unregulated. Limiting or eliminating fossil fuel burning from stoves indoors would support reductions in long-term and acute residential NO₂ exposures. Using the exposure maps generated through this study, decision makers can prioritize regions and even communities down to the ZIP-code-level that would benefit most from a transition to cleaner cooking technologies among other air quality interventions.