A new study links how many hurricanes form worldwide to air pollution levels.

Wednesday's study from the National Oceanic and Atmospheric Administration says cleaner air in Europe and the United States is helping trigger a dramatic increase in the number of Atlantic hurricanes. And at the same time dirtier air in China and India means fewer tropical cyclones in the Pacific.

That's because aerosol pollution, which is the small sulfur particles and droplets that make it hard to breathe and see, cools the air, slightly counters global warming from greenhouse gas. Less air pollution means warmer air, warmer water and more storms.