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Last month, at a vast composting yard owned by a Northern California waste and recycling company, Recology, I watched a load of lawn and food scraps from San Francisco residents get fed into a sorting machine. A spinning cylinder resembling a supersized cheese grater sifted out tidbits like lime wedges and grass clippings and spit the chunkier items onto a platform, where a worker in a neon vest plucked out plastic bags and an aerosol can of glass cleaner-just a few of the hundreds of pieces of contraband that he’d cull that day. I asked if he ever let anything slip by. “Sometimes,” he said with a sheepish smile. I later ran my hand through a ripened compost pile and felt little pieces of glass and plastic mixed in with the fertile humus.

As thousands of cities have begun composting yard waste and hundreds more begin collecting food scraps on a large scale, new questions are emerging about what kinds of things make their way into compost and whether any of them pose a threat to humans and the environment. Federal laws do not require compost to be screened for contaminants, of which plastic and glass are only the most visible. Random tests of compost used in organic agriculture have occasionally turned up elevated levels of lead and traces of pesticides. Last month, the US Composting Council, the industry’s trade group, warned its members to watch out for grass clippings laced with Imprelis, a new weed killer from DuPont that does not easily break down in compost piles.