Health effect of mixtures of ozone, nitrogen dioxide, and fine particulates in 85 US counties

Despite substantial improvements in ambient air quality in the past decades, ozone (O₃), nitrogen dioxide (NO₂), and fine particulate matter (PM_2.5) remain as concerns. As these pollutants exist as mixtures in ambient environments, some combinations of these pollutants may be more harmful to human health than other combinations. Identifying harmful pollutant mixtures can help develop multi-pollutant control strategies to better protect health. Current methods exhibit limitations in identifying harmful mixtures. We aim to identify harmful compositions of three-pollutant mixtures in 85 US counties during 1999-2010. We developed a new method called PANCAKE to quantify O₃-NO₂-PM_2.5 mixtures. O₃-NO₂-PM_2.5 mixtures are categorized into 27 composition types based on combinations of different O₃, NO₂, and PM_2.5 levels. We identified harmful compositions by estimating the effect of each composition of O₃-NO₂-PM_2.5 mixture compared to the reference composition on cardiovascular admissions among Medicare patients. We found that a mixture with relatively low levels of some pollutants combined with relatively high levels of other pollutants can be equally or more harmful than a mixture with high levels of all pollutants. Eight out of the 27 composition types, often with NO₂ levels > 17.3 ppb and PM_2.5 levels > 8.8 μg/m³ combined with any levels of O₃, were associated with significantly increased cardiovascular admission rates compared to the reference composition. These harmful compositions overall occurred in about 40-50% of days in winter, metropolitan areas, or the East North Central region. Mixture composition plays an important role in determining health risks and may be worth considering when developing air pollution control strategies.