For decades the primary source of urban air pollution has been cars. Now many cities are reaping the clean air rewards of automobile pollution controls, but we've overlooked smaller contributors that, collectively, are now as much of a problem as transport.

Many widely used chemicals release volatile organic compounds (VOCs). Some advertise the fact with the familiar fresh paint smell, but others can easily go unnoticed. It's long been known VOCs can be a major problem indoors, causing conditions such as “sick building syndrome”. However, the great outdoors was thought to be a different matter. After all, Americans go through about 15 times as much fuel in cars as they use petroleum-based chemicals for everything else, and it was naturally expected the polluting effects would be proportionate.

However, science exists to check these sorts of assumptions, and that is what Dr Brian McDonald of the National Oceanic and Atmospheric Administration (NOAA) did. The relative importance of different sources depends on location and the form of pollution you are looking at. Focusing on PM_2.5 particles, McDonald found that pesticides, photocopier toner, paints, and similar sources actually account for twice as much of the air pollution in Los Angeles as cars.

"As transportation gets cleaner, those other sources become more and more important," McDonald said in a statement. "The stuff we use in our everyday lives can impact air pollution."

VOCs can cause headaches and directly damage the liver and kidneys, but they also react with nitrogen oxides to form particulates of the size that get deep into our lungs where they can cause respiratory diseases and lunch cancer. VOCs can also react to produce ozone, which, while an essential shield for the planet in the stratosphere, is a damaging pollutant in the lower atmosphere.

McDonald reported in Science that estimates of VOCs from common chemicals are two to three times lower than the reality in major American cities.

Co-author NOAA's Dr Jessica Gilman explained the larger contribution from certain products comes from the way we use them. "Gasoline is stored in closed, hopefully airtight, containers and the VOCs in gasoline are burned for energy," Gilman said. "But volatile chemical products used in common solvents and personal care products are literally designed to evaporate. You wear perfume or use scented products so that you or your neighbor can enjoy the aroma. You don't do this with gasoline.”

With air pollution killing 5.5 million people a year, identifying the sources matters. In much of the world vehicles and coal plants almost certainly remain the largest sources of pollution, but where progress has been made in tackling these, the authors argue, it will be essential to expand the focus if major improvements are to continue.