Sars patients are more than twice as likely to die if they live in areas of high pollution, researchers report.
Sars caused widespread concern
US and Chinese scientists examined death rates in five regions of China, where 5,327 cases of Sars have been registered and 349 people have died.
In regions with low air pollution, the death rate was 4.08%, but where pollution was high it rose to 8.9%.
However, a UK doctor who spoke to BBC News Online expressed scepticism about the impact of air pollution.
Symptoms of Sars:
Shortness of breath
Loss of appetite
The new research is published in Environmental Health: A Global Access Science Source.
It finds that the lowest death rate among people with Sars was recorded in Guangdong province, which has a low level of pollution.
Shanxi, Hebei and Beijing, where pollution levels are moderate, recorded a higher rate of deaths.
While the highest death rate was recorded in Tianjin, where air pollution was high.
The researchers suggest that: "long-term or short-term exposure to certain air pollutants could compromise lung function, therefore increasing Sars fatality."
In cases where Sars is fatal, it is thought that the infection stimulates a severe inflammation of the lungs, leading to tissue damage, serious respiratory problems, and ultimately multiple organ failure.
The researchers acknowledge they were not able to take into account the socioeconomic status or the smoking habits of the Sars patients. Nor did they consider the treatment that the patients were given.
All of these may have contributed to the patients' outcome.
However, the two regions with the highest case fatality rates were Beijing and Tianjin.
The researchers suspect that patients would probably have received better clinical support in these areas.
If this is the case, then air pollution may play an even greater role in increasing death rates than their data suggests.
Researcher Professor Zuo-Feng Zhang said air pollution may damage the lining of the lung, making it more vulnerable to the inflammation caused by Sars infection.
He said: "If there is another outbreak of Sars, our findings suggest that caregivers need to pay close attention to Sars patients with exposure to airborne toxins in the living and working environments, including cigarette smoking and occupational exposure to airborne toxins, which could leave some individuals at greater risk of death from the illness than others."
Professor Roy Anderson, an expert in Sars from Imperial College, London, told BBC News Online he was not surprised by the findings.
"Air pollution does decrease lung function, causing problems with respiration, and therefore it does not surprise me that it is associated with slightly raised mortality for Sars," he said.
However, he warned that a thorough analysis had to take into account other factors, such as whether patients had a pre-existing heart condition.
Age also appeared to be an important factor, he said. No deaths have been recorded among Sars patients under 30, but among those aged over 80 the death rate is 80%.
Dr Paul Wilkinson, from the London School of Hygiene and Tropical Medicine, told BBC News Online he was unconvinced that air pollution was a significant factor.
"China does have some pretty high levels of air pollution in some of its cities, but it would be a little surprising if relatively modest differences in air pollution made a big difference to the effect of the acute, severe inflammatory reaction which can be triggered by Sars.
"However, air pollution is known to increase mortality from respiratory and cardiac causes, so I suppose the theory is plausible."
A team from the University of California in Los Angeles collaborated on the study with scientists from Jiangsu Provincial Center for Disease Control and Prevention, and Fudan University School of Public Health.