Long-term nitrogen dioxide exposure and cause-specific mortality in the U.S. Medicare population.

Background: Since 1971, the annual National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO) has remained at 53 ppb, the impact of long-term NO exposure on mortality is poorly understood.

Objectives: We examined associations between long-term NO exposure (12-month moving average of NO) below the annual NAAQS and cause-specific mortality among the older adults in the U.S.

Methods: Cox proportional-hazard models were used to estimate Hazard Ratio (HR) for cause-specific mortality associated with long-term NO exposures among about 50 million Medicare beneficiaries living within the conterminous U.S. from 2001 to 2008.

Results: A 10 ppb increase in NO was associated with increased mortality from all-cause (HR: 1.06; 95% CI: 1.05-1.06), cardiovascular (HR: 1.10; 95% CI: 1.10-1.11), respiratory disease (HR: 1.09; 95% CI: 1.08-1.11), and cancer (HR: 1.01; 95% CI: 1.00-1.02) adjusting for age, sex, race, ZIP code as strata ZIP code- and state-level socio-economic status (SES) as covariates, and PM exposure using a 2-stage approach. NO was also associated with elevated mortality from ischemic heart disease, cerebrovascular disease, congestive heart failure, chronic obstructive pulmonary disease, pneumonia, and lung cancer. We found no evidence of a threshold, with positive and significant HRs across the range of NO exposures for all causes of death examined. Exposure-response curves were linear for all-cause, supra-linear for cardiovascular-, and sub-linear for respiratory-related mortality. HRs were highest consistently among Black beneficiaries.

Conclusions: Long-term NO exposure is associated with elevated risks of death by multiple causes, without evidence of a threshold response. Our findings raise concerns about the sufficiency of the annual NAAQS for NO.