Carbon monoxide emitted by traffic may be harmful even at levels far below those considered safe by government standards, a Yale study has found.

An odorless, tasteless air pollutant released by cars in traffic, carbon monoxide is associated with an increased risk of hospitalization due to cardiovascular illness among the elderly, according to a study led by School of Forestry & Environmental Studies professor Michelle Bell, published last week.

“Higher levels of carbon monoxide were associated with higher risk of hospitalizations for cardiovascular heart disease,” Bell said in an e-mail message. “This evidence indicates that exposure to current carbon monoxide levels may still pose a public health threat.”

The study was done by comparing hospitalization records with air pollution records in 126 urban U.S. counties and analyzing the correlation between same-day carbon monoxide levels and an increased risk of hospitalization for cardiovascular disease. The team adjusted for other traffic pollutants, such as nitrogen dioxide and fine particles that may also have had bearing on the subjects’ general health.

Previous studies have focused on the health effects of much higher levels of carbon monoxide. This study, however, found that dramatically smaller quantities released in everyday traffic could have significant health repercussions. The effects of low levels of carbon monoxide concentrations on health had not previously been adequately studied, Bell said.

The Environmental Protection Agency has declared a limit of 35 parts per million in its National Ambient Air Quality Standard, but this study found levels less than 1 part per million to still be potentially damaging.

“The levels of CO in the country are generally quite low and are lower than the National Ambient Air Quality Standards,” Roger Peng, a professor at the Johns Hopkins Bloomberg School of Public Health and co-author, said. “Even at low levels, there are substantial health effects.”

The study was partially funded by the Environmental Protection Agency, which is in the process of reviewing the scientific evidence for the health effects of carbon monoxide.

When reached Tuesday afternoon, Cathy Milbourn, the EPA representative for air quality, said she was unaware of major changes of the levels of carbon monoxide specified by the EPA National Ambient Air Quality Standards.

If the study led to administrative changes, they would be slow to come, Peng said.

“Any change in EPA policy is a complex process, and there is a lot of evidence that needs to be reviewed,” he said.

The primary source of carbon monoxide is traffic exhaust, so concentrations in suburban and rural areas are likely to be lower and less dangerous than in urban areas of high traffic density, Peng said.

Of the 126 counties studied, New Haven’s mean carbon monoxide concentration, at 1.1 parts per million at the peak hour of particulate concentration, was slightly lower than those of other counties studied. Jefferson, Ala., had the highest mean concentration, at 4.3 parts per million.