The study traces death rates among various British electoral wards from the early 1980s through the late 1990s.

When the study began, more than 2 million people lived in those areas. As the years passed, more than 420,000 died of various causes.

The researchers included Paul Elliott, FMedSci, of Imperial College London. They focused on two types of air pollution: black smoke, which comes from burning coal, and sulfur dioxide.

During the study period, air pollution levels dropped. But even so, adults aged 30 and older living in areas with higher levels of air pollution had higher death rates, especially from respiratory diseases, according to the study.

The researchers considered the fact that air pollution may be worse in low-income neighborhoods. That didn't change the results.

"Our findings from extensive British data add to the evidence that air pollution has long-term effects on mortality, and point to continuing public health risks even at the relatively lower levels of black smoke and sulfur dioxide that now occur," write Elliott and colleagues.

However, the study has some limits.

The researchers didn't check the medical histories of the people living in the neighborhoods that were studied. So the study doesn't prove that air pollution directly caused any deaths.