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Fish, shrimp and other seafood catches from the Gulf of Mexico is being ground up and put under the microscope to hunt for minute traces of oil. - far more reassuring than that sniff test that made all the headlines. Health regulators are working to create a test for dispersant contamination. More Gulf waters are reopening to commercial hauls as tests show little hazard from oil. The oil contaminants of most health concern - potential cancer-causing substances called polycyclic aromatic hydrocarbons, or PAHs - show up in other everyday foods, too, such as grilled meat.

Here are some questions and answers about Gulf seafood safety:

Q: What are PAHs?
A: They're common pollutants from oil, vehicle exhaust, wood-burning fires and tobacco smoke. They can be in food grown in polluted soil and form in meat cooked at high temperatures.

Q: How does the government decide it's safe to reopen fishing waters?
A: Seafood testing begins when there's no longer visible oil in a particular area. First, inspectors smell samples for the slightest whiff of oil. Step 2 is chemical testing at the Food and Drug Administration, NOAA, or state laboratories. To reopen seafood harvesting, the samples must test below FDA-set "levels of concern" for 12 different PAHs, based on how much someone would have to eat for a potential health risk, and how much of each food fairly heavy seafood consumers tend to eat in a month.

Q: With so much oil in the Gulf, how could fish emerge untainted?
A: Commonly consumed fin fish - like grouper, snapper and tuna - rapidly metabolize those PAHs. That's been known for years and tracked during other oil spills, and the reason that fishing is being allowed first in reopened waters. The safe limit for the PAH naphthalene is 3.3 parts per billion. The highest levels found in recently reopened waters off the Florida panhandle were well below that, 1.3 ppb, mostly in red snapper.

Q: Why haven't crabs and oysters been cleared?
A: They're the slowest metabolizers, plus crabs require an extra testing step that FDA hasn't finished. Oysters are probably the best absorbers of oil, as they take in both droplets and dissolved oil, according to Carys Mitchelmore, an aquatic toxicologist at the University of Maryland Center for Environmental Science. Most oyster testing is just beginning, so stay tuned, although the FDA recently cleared some from Alabama that contained less than a quarter of the total PAH limit of 66 parts per million.

Q: But what about that controversial dispersant - are the feds testing for it?
A: Not yet; they're still developing a good test.

Q: So why do they say dispersant isn't a seafood threat?
A: Some dispersant chemicals are FDA-regulated ingredients in skin creams and even foods. FDA contends the stronger cleansing ingredients under question degrade too quickly in water to accumulate in fish flesh. In experiments under way in Texas and Alabama, federal scientists are dumping dispersant into tanks full of shrimp, oysters and crabs to try to detect even minute levels

Health regulators will continue to test as long as it's needed. (Wash Post, 8/16/2010)