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

Concentrations of nitrogen dioxide (NO₂) measured outdoors using satellites and ground-level stations across the United States are regularly used to estimate NO₂ exposures and disease burdens. In contrast, exposures attributable to sources of NO₂ indoors are neither systematically monitored nor estimated. Here, to our knowledge we produce the first nationwide, ZIP-code-level estimate of total residential NO₂ exposure that integrates both outdoor sources and the primary indoor source-gas- or propane-burning stoves. To estimate exposure by ZIP code, we combine our measurements of indoor NO₂ emissions and concentrations in more than 15 cities across seven regions with outdoor NO₂ concentrations and comprehensive housing stock data for 133 million residential dwellings and statistical samplings of occupant behavior. We estimate average total residential long-term NO₂ exposure across the United States to be ∼10 ppbv for people who own a gas stove (∼18 ppbv or more for households in the top 5% of gas burned while cooking) and ∼8 ppbv from total outdoor exposure for those with electric stoves, which cause no additional NO₂ exposure. Across the United States, the NO₂ exposure of ∼22 million people would fall below the World Health Organization (WHO)'s long-term exposure guideline (10 µg/m³ or 5.2 ppbv) if they reduced or stopped cooking with gas or propane. Gas and propane stoves are also responsible for virtually all (>99%) of the residential exceedances of the WHO's 1-h-averaged air quality guideline across the United States. Gas and propane stoves are a substantial source of residential NO₂ exposure even when compared with all outdoor sources combined.