Raising animals for food contributes to air and water pollution. Manure produces toxic hydrogen sulfide and ammonia which pollute the air and leach poisonous nitrates into nearby waters. The USDA estimates that livestock produces 500 million tons of manure annually—three times what humans produce. [32] Runoff laden with manure is a major cause of "dead zones” in 173,000 miles of US waterways, [32] including the 7,700-square-mile dead zone in the Gulf of Mexico. [115] People living near CAFOs often have respiratory problems from hydrogen sulfide and ammonia air pollution. A peer-reviewed 2006 study of Iowa students near a CAFO found 19.7% had asthma - nearly three times the state average of 6.7%. [166]