Air pollution has been linked to a number of detrimental effects on the environment and human health over the years. But it may also have produced one unintended positive impact: lowering the frequency of tropical storms in the Atlantic, including hurricanes. That's at least the conclusion of researchers at the Met Office, the UK's national weather service, who examined nearly 200 years-worth of air pollution and storm data, including several years into the future via a computer model.

They found that air pollution particles produced by industrial activity in the US and Europe, called aerosols, reduced the frequency of tropical storms over most of the 20th century. But now, air pollution levels have fallen fairly dramatically in these regions in recent years thanks to increased regulations and a shift to cleaner-burning fuels. Unfortunately, more hurricanes appear to be forming as a result, according to the study, published online yesterday in the journal Nature Geoscience.

Reductions in air pollution have "contributed to recent increases in hurricane numbers."

The reason for the counterintuitive effect has due to with the impact of aerosols on cloud formation. Specifically, more aerosols in the atmosphere lead to whiter, more reflective clouds. These clouds reflect more heat from the sun back into the atmosphere, away from Earth, cooling the Atlantic and making conditions in the ocean less ideal for hurricanes and tropical storms. But, "since the introduction of the clean air-acts in the 1980s, concentrations of aerosols over the North Atlantic have reduced and model results suggest that this will have contributed to recent increases in hurricane numbers," said Nick Dunstone, the lead scientist behind the study. Still, Dunstone noted that such regulations have improved air quality and reduced health risks, so the fact that hurricane formation rates may increase in the future is at worst, a mixed outcome.