WASHINGTON -- Fish, shrimp and other catches from the Gulf of Mexico are being ground up to hunt for minute traces of oil in what's considered unprecedented safety testing -- sort of a "CSI" for seafood that's far more reassuring than the sniff test that made all the headlines.

And while the dispersant that was dumped into the massive oil spill has consumers nervous, health regulators contend there's no evidence it builds up in seafood -- although they're working to create a test for it, just in case.

"We're taking extraordinary steps to assure a high level of confidence in the seafood," said Jane Lubchenco, administrator of the National Oceanic and Atmospheric Administration.

More Gulf waters are reopening to commercial hauls as tests show little hazard from oil. Louisiana's fall shrimp season kicked off Monday. Yet it's too soon to know what safety testing will satisfy a public so skeptical of government reassurances that even local fishermen voice concern.

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. Low levels also are in seafood sold from other waters.

Here are some questions and answers about Gulf seafood safety:

Question: What are PAHs?

Answer: 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. NOAA research found that Alaskan villagers' smoked salmon contained far more PAHs than shellfish tainted by the Exxon Valdez spill.

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 PAHs, based on how much someone would have to eat for a potential health risk.

Q: What if fishermen illegally fish in closed waters?

A: The government is patrolling those waters, doing dockside sampling and stepping up inspections at seafood processors.

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.

For example, the limit in fish of the PAH named benzo(a)pyrene is 35 parts per billion. In recently reopened waters off the Florida panhandle, levels were below 1 ppb.

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 droplets and dissolved oil, said 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.