In these days of hybrid cars and carbon credits, it is common knowledge that substances exhaled by autos and coal plants are harmful to our respiratory system. What may be surprising is the degree to which they may harm the brain—in some instances, as much as exposure to lead. A recent string of studies from all over the world suggests that common air pollutants such as black carbon, particulate matter and ozone can negatively affect vocabulary, reaction times and even overall intelligence.

The most recent of these studies found that New York City five-year-olds who were exposed to higher levels of urban air pollutants known as polycyclic aromatic hydrocarbons (PAH) while in the womb exhibited an IQ four points lower than those subjected to less PAH. Alarmingly, “the drop was similar to that seen in exposure to low levels of lead,” says epidemiologist Frederica Perera, director of the Columbia Center for Children’s Environ­mental Health and head author of the study, in which mothers wore personal air monitors during their pregnancy. The IQ change was enough of a dip to affect school per­formance and scores on standardized tests.

“These weren’t even superimpressively high levels of pollution,” Perera says. “The levels we measured in our study are comparable to those in other urban areas.” Most PAH pollutants come from motor vehicle emissions, especially diesel- and gas-powered cars and trucks, and from the burning of coal. (Tobacco smoke is another source, so the researchers did not enroll smokers in the study and corrected for secondhand smoke exposure.)

But children’s growing brains are not the only ones affected by this dirty air. A 2008 study in 20- to 50-year-olds conducted jointly by the schools of public health at Harvard University and the University of North Carolina at Chapel Hill pinpointed ozone-related reductions in attention, short-term memory and reaction times equivalent to up to 3.5 to five years of age-related decline.

What’s to be done about these brain-harming pollutants? “It’s not a mystery how to reduce them—we need better policies on traffic congestion and technologies for alternative energy and energy efficiency,” Perera says. Fortunately, there are also more immediate ways to reduce your exposure to the toxic chemicals, such as limiting outdoor physical activity on smoggy days. Ozone alerts and air-quality reports have become a routine part of the morning weather forecast and also appear on sites such as “Depending on where you live, it becomes a good idea to pay attention to air quality before exercising outdoors,” says Lisa Jackson, administrator of the U.S. Environmental Protection Agency. “There is also some benefit to dialing down the intensity if you can’t avoid exercising outside—for example, walking instead of running.”

Another smart move: avoid walking, running or bike riding on major streets with heavy bus, truck or car traffic whenever possible, Jackson says. Until emissions controls and other EPA policies begin to significantly impact the levels of traffic-related pollutants in the air around us, bathing our brains in as little of the stuff as possible may be our—and our children’s—best bet.