More expensive than it looks: Polluted air is an often unmentioned cost of mining gold.

Warnings about mercury in fish and seafood have gotten plenty of attention in recent years. But where does all that mercury come from in the first place? Apparently, the largest sources of mercury emissions into our air and water are going all but unnoticed. A new study published in the Journal of Environmental Monitoring has found that mercury in the atmosphere is an oft-ignored form of air pollution, especially in urban areas where concentrations can reach dangerously high levels. The U.S. Environmental Protection Agency (EPA) has been lax in its enforcement of mercury-pollution standards over the past decade, exempting major polluters. Mercury is a neurotoxin that can harm the developing brains of children and infants; in adults, exposure can lead to memory loss and affect fertility and blood pressure.

Here are the biggest emitters of mercury into the environment:

1. Coal-fired power plants. Mercury exists naturally in coal, making coal-fired power plants the largest source of mercury pollution in this country. Coal accounts for nearly 50 percent of the electricity generated in this country—and almost 50 tons of mercury emissions annually.

What you can do: Contact your electricity provider to see if buying green power is an alternative in your area. If it is, opt to get your energy from a less polluting source, such as wind, solar, or hydroelectric power.

2. Cement kilns. According to the nonprofit law firm Earthjustice, all the cement kilns in the U.S. combined pump out roughly 23,000 pounds of mercury every year. The mercury comes from coal, which is used to fuel the cement-manufacturing process, as well as limestone, another natural source of the heavy metal. The group released a report last July finding that, individually, some cement kilns emit nearly one and a half times more mercury than the most polluting coal-fired power plants. But because there are fewer kilns, they account for lower levels of atmospheric mercury overall than coal plants.

What you can do: Cleaning up this industrial process will take more political power than individual action, but you can lobby your local officials to put pressure on nearby cement kilns to clean up their acts. See if you live near a cement production facility by searching Earthjustice’s interactive map. If you do, support local environmental groups working for stricter mercury controls. You can also buy a water filter that’s certified to remove mercury from your drinking water, which will cut down on some of your exposure.