Carbon Monoxide Increases Heart-Related Hospitalizations

Ruzanna Harutyunyan's picture

Levels of ambient carbon monoxide (CO) in the air well below accepted environmental standards are associated with an increased risk of hospital admissions for heart problems among the elderly, according to a study reported in Circulation: Journal of the American Heart Association.

In one of the first nationwide studies of ambient low-level carbon monoxide emissions and cardiovascular disease (CVD), a 1 part per million (ppm) increase in daily one-hour exposure was associated with a 0.96 percent increase in the risk of hospitalization among people over age 65.

The increased risk persisted at extremely low CO levels of less than 1 ppm, suggesting that the detrimental effects of even short exposure to low levels of CO or other traffic related pollutants pose an under-recognized health risk to seniors, said Michelle L. Bell, Ph.D., lead researcher of the study.

A concentration of 1 ppm corresponds to 1 part pollutant per 1 million parts of breathable air. As a liquid, it’s equivalent to about one drop of water in a 13-gallon trash can.

“This may be evidence of an association between short-term exposure to ambient carbon monoxide and risk of cardiovascular disease hospitalizations, even at levels well below current U.S. health-based regulatory standards,” said Bell, associate professor of environmental health at Yale University’s School of Forestry and Environmental Studies in New Haven, Conn.

“This evidence indicates that exposure to current carbon monoxide levels may still pose a public health threat, particularly for persons with CVD.”

Current CO levels in the United States are low and well below the U.S. Environmental Protection Agency’s (EPA) health-based National Ambient Air Quality Standard of 35 ppm for one-hour daily maximum in almost all areas. Carbon monoxide is a tasteless, odorless gas in the outdoor atmosphere that’s generated by traffic exhaust and other combustion.

Researchers based their findings on a review of hospital records for 9.3 million Medicare enrollees and air pollution levels gathered between 1999 and 2005 in 126 urban counties around the United States.


Most previous evidence linking CO and CVD has come from controlled studies involving CO inhalation at higher concentrations. Prior research on short-term exposure to ambient CO levels and CVD hospitalizations has yielded inconsistent results, and the potential role of other pollutants or whether CVD rates increase with low-level exposure has not been adequately studied.

“We found evidence that the association between CO and risk of cardiovascular hospitalizations persisted at daily exposure levels of under 1 ppm,” Bell said.

Researchers examined hospital admission records using different exposure models adjusted for other traffic-related pollutants, including nitrogen dioxide, fine particles and elemental carbon.

“We found a positive and statistically significant association between same-day carbon monoxide levels and an increased risk of hospitalization for multiple CVD outcomes, including ischemic heart disease, heart rhythm disturbances and heart failure, as well as cerebrovascular disease and total CVD,” Bell said.

“One critical finding of our research is the perhaps unexpectedly strong effect observed at current ambient levels. Additional research is needed to disentangle the effects of CO from other traffic-related pollutants such as fine particles.”

In an accompanying editorial on this study and one on smoking, Annette Peters, Ph.D., from the German Research Center for Environmental Health, Institute of Epidemiology, notes that the research “makes an important contribution of the assessment of the role of the environment for cardiovascular health.” Together, the studies:

• highlight the consistently emerging evidence that both indoor and outdoor air quality is a modifiable risk factor for cardiovascular disease;

• provide consistent evidence for health effects of particulate matter;

• note that the health effects of low dose smoking, secondhand smoke and air pollution are probably still underestimated by decision makers around the world; and

• suggest that air quality regulation should include regular reassessment of emerging and existing air quality measures in addition to a standard for particulate matter smaller than 2.5 ?m (PM2.5).