Quick, what's the biggest single source of mercury pollution in the Bay Area? If you guessed something like the Chevron facility that caught fire in Richmond last month, you'd be wrong.

AirPollution_ Mark_H_Anbinder_Flickr_02142012 — A smokestack belching smoke. (Photo: Mark Anbinder/Flickr)

It turns out that Lehigh Permanente, a Cupertino company that mines lime and uses it to make cement, put about 260 pounds of mercury into the air in 2011. That's more than any other single facility, according to Aaron Richardson, a spokesman for the Bay Area Air Quality Management District.

And mercury is only the beginning. On Wednesday, the BAAQMD will hold a public hearing on a proposal to tighten restrictions on the amount of mercury, dust, ammonia, hydrocarbons, dioxins, nitrogen oxides and other pollutants the plant releases.

The plant has already been cutting its mercury emissions; output is down from 1,200 pounds in 2005. The new regulations would chop the total by another 95 percent, Richardson said.

The U.S. Geological Survey web site says that "mercury can be a threat to the health of people and wildlife in many environments that are not obviously polluted." But monitoring at the plant has showed no cause for concern, Richardson said. "Our sense is that at the moment the impact on the community is not significant."

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More mercury comes from the Bay, where about 400 pounds of mercury runoff evaporates into the air every year, said Richardson.

Still, local residents worry. "It's definitely having an impact on people's health," said Cynthia Hayes-Rapp, who lives near the plant. Advocates of further regulation have organized themselves as Bay Area for a Clean Environment and say they've collected more than 1800 signatures calling for more stringent rules than those in the proposed regulations, including a requirement for a central smokestack that they think is the best way to filter toxins from the plant's emissions.

A public relations consultant for Lehigh, which is owned by the German company Heidelberg Cement, said no one was available to talk about the proposed regulations.

Instead he provided a company newsletter saying that Lehigh was reducing its emissions, and that Cupertino has lower particulate matter levels in its air than San Jose, San Francisco, Oakland and 10 other Bay Area cities.

Here's a passage from the newsletter:

Notably, they found that Cupertino air quality levels were well below all applicable State and national Ambient Air Quality Standards for gaseous criteria pollutants including ozone, CO, SO2 and NO2.

So why is the BAAQMD considering further regulation? The BAAQMD's Richardson cited community pressure. He also pointed out that the U.S. Environmental Protection Agency is also considering tough new regulations on cement plants, and BAAQMD wanted its regulations to match those.

Representatives of the plant, as well as neighbors, will get a last chance to present their points of view during public comments before the BAAQMD votes on the proposed regulations Wednesday.

The hearing is set for 9:45 a.m. in the 7th Floor Board Room at the Air District Headquarters, 939 Ellis Street, San Francisco, California.

If you can't make it in person, you can follow the whole thing on webcast.

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More Regulation Considered For Bay Area's Biggest Mercury Polluter, Which Is...