Air pollution isn't only bad for people's hearts and lungs, new research from France shows.

After peak pollution days, more people visit a primary care doctor for upper and lower respiratory tract disease, headache, feelings of weakness, skin rash and conjunctivitis, Dr. Sophie Larrieu of the French Institute of Public Health and Surveillance in Bordeaux and her colleagues found. "This is one more convincing argument for promoting all measures aimed at reducing pollutant emissions, on both the individual and collective levels, even in moderately polluted areas," Larrieu and her team say.

The link between dirty air and illness and death from cardiorespiratory causes is clear (for example hospitalization for heart attack or asthma exacerbation), the researchers point out, but little is known about whether pollution might trigger less severe but more common health effects.

To investigate, they looked at patient visit data gathered by 60 general practitioners in Bordeaux between 2000 and 2006, comparing it to information on local air pollution levels for the same time period. The "moderately polluted" area they studied encompasses 22 cities and has a population of about 600,000.

There were 895,710 visits to the network of doctors during the study period. After a rise in particulate matter or nitrogen dioxide levels, the researchers found, more people visited the doctor for upper and lower respiratory tract disease. Rises in both pollutants, as well as in ozone levels, were also associated with increases in visits for headache or asthenia (a generalized sense of weakness). Increases in particulate matter and in ozone boosted visits for skin rash or conjunctivitis.

Effects often lasted for several days after the rise in pollution; nitrogen dioxide showed a "much higher" effect on conjunctivitis and skin rash when the researchers looked at delayed effects, and the effects of increases in nitrogen dioxide on respiratory diseases lasted for two weeks.

But there was no relationship between levels of any of the pollutants and the number of people seeking care for asthma. This may be because asthma patients often treat themselves, the researchers suggest, or may go directly to an emergency room when they can't control their symptoms.

"These results suggest that considering only the most severe effects of air pollution leads to underestimation of its impact on public health," they conclude.