Visits to the emergency department (ED) for heart- and lung-related illnesses climb as air gets dirtier, a new analysis of data from seven Canadian cities shows.

Dr. David M. Stieb of Health Canada in Ottawa, Ontario and his colleagues found a rise in ED visits for cardiac conditions along with elevations in carbon monoxide and nitrogen dioxide levels, while increases in ozone levels were followed by a rise in ED visits for respiratory problems.

There is strong evidence linking high levels of air pollution to hospitalizations of relatively ill people, but less is known about how healthy people fare when air pollution worsens, Stieb and his team note in their report. They decided to look at ED visits because these represent a wider — and healthier — sample of the population than hospitalizations, the researchers say, because fewer than half of people seen in the ED are ill enough to be admitted to the hospital.

The researchers analyzed data from the late 1990s and early 2000s for ED admissions at 14 different hospitals across Canada in relation to air pollution records from the National Air Pollution Surveillance system, as well as archived weather data for the same time period.

The most "consistent" links they found were between carbon monoxide and nitrogen dioxide and visits for heart attack or angina and heart failure, and between ozone levels and visits for asthma and chronic obstructive pulmonary disease.

The relationships were stronger in the warmer months of the year, especially for air pollution and ED visits for asthma, which were three to four times greater between April and September compared to the year-round average — probably because people spend more time outdoors in milder weather, increasing their pollution exposure.

The researchers conclude: "These results add further weight to arguments regarding the role of air pollution in contributing to adverse health events, and imply that interventions to reduce these pollutants are warranted in an effort to reduce cardio-respiratory ED visits."