Background— Evidence on risk of cardiovascular disease (CVD) hospitalization associated with short-term exposure to outdoor carbon monoxide (CO), an air pollutant primarily generated by traffic, is inconsistent across studies. Uncertainties remain on the degree to which associations are attributable to other traffic pollutants and whether effects persist at low levels.
Methods and Results— We conducted a multisite time-series study to estimate risk of CVD hospitalization associated with short-term CO exposure in 126 US urban counties during 1999–2005 for >9.3 million Medicare enrollees aged ≥65 years. We considered models with adjustment by other traffic-related pollutants: NO2, fine particulate matter (with aerodynamic diameter ≤2.5 μm), and elemental carbon. We found a positive and statistically significant association between same-day CO and increased risk of hospitalization for multiple CVD outcomes (ischemic heart disease, heart rhythm disturbances, heart failure, cerebrovascular disease, total CVD). The association remained positive and statistically significant but was attenuated with copollutant adjustment, especially NO2. A 1-ppm increase in same-day daily 1-hour maximum CO was associated with a 0.96% (95% posterior interval, 0.79%, 1.12%) increase in risk of CVD admissions. With same-day NO2 adjustment, this estimate was 0.55% (0.36%, 0.74%). The risk persisted at low CO levels <1 ppm.
Conclusions— We found evidence of an association between short-term exposure to ambient CO and risk of CVD hospitalizations, even at levels well below current US health-based regulatory standards. This evidence indicates that exposure to current CO levels may still pose a public health threat, particularly for persons with CVD.