The mercury levels they've measured are in parts per trillion, orders of magnitude less than the more worrisome parts per million levels tracked by the EPA and FDA. So the fog is not dangerous. But it does contain four times as much mercury as local rainwater does.

 

And when the fog rolls in, it collects on redwood leaves, and drips to the ground, where it enters the food chain. Preliminary data suggest that during the foggy season, wolf spiders in the area see their mercury levels multiply by five. The findings were presented at a meeting of the American Geophysical Union in San Francisco.

 

Researchers say the mercury in coastal fog probably comes from deep ocean waters, which well to the surface every summer. But its original source may be coal-burning power plants, or pollution from mercury mines dating back to the gold rush. Either way, it's a reminder that whatever we dump into the environment will probably pay us a return visit.