Related topics: •  Digestive Disorders Center •  Quiz: How Much Do You Know About Food Safety? •  Special Report: Sushi Worms

Fish lovers can consider themselves lucky. Truly fresh seafood is more widely available in the United States now than it's ever been. But like red meat and poultry, fish can be risky if it's not prepared properly. When people get sick from eating undercooked chicken or beef, it's typically due to bacterial contamination. (During processing, bugs from inside the animal can wind up on the outside.)

With fish, particularly salmon and rockfish, the concern is parasitic worms. Virtually all the raw fish served in sushi bars has been frozen long enough to make it safe to eat. However, fresh fish bought in a market or newly caught warrants careful handling.

Fish is safely cooked when the flesh flakes easily and the inside no longer looks rare (that is, it's opaque throughout). Technically speaking, that's an inside temperature of 145 degrees, maintained for at least 15 seconds. A modern quick-read food thermometer should be used.

Chefs at many upscale restaurants insist on offering never-been-frozen fish as a matter of professional pride. And they often lightly sear it on the outside, then serve it rare -- salmon especially. People who are concerned about parasites can always tell the waiter that they'd like their fish cooked all the way through.

Not to worry. The raw salmon brined and smoked to make commercial lox has been frozen, which knocks off any unwelcome guests. (The smoking process usually doesn't.)

Freezing fish at -4 degrees for 7 days will kill parasites and prevent illness, according to the Food and Drug Administration. That's no problem in commercial freezers. Unfortunately, household refrigerator freezers generally cycle between 5 and 10 degrees.

Fish caught in lakes and streams is more likely to have parasites than the farmed fish often sold in supermarkets, and campfires aren't usually as hot as stoves. On fishing trips people should be diligent when cooking their catch.

Individuals with weak immune systems, especially those with liver disease, should not eat raw oysters or other raw shellfish.

Nearly all fish and shellfish contain very small amounts of mercury. According to the Environmental Protection Agency and the FDA, these amounts aren't high enough to be a health hazard for most people, though some fish contain more mercury than others and should be avoided by certain groups. In a joint statement released in 2004, the EPA and the FDA advised that women who may become pregnant, women who are already pregnant, nursing mothers, and young children should avoid eating shark, swordfish, king mackerel, or tilefish, and should eat no more than two six-ounce cans of albacore tuna per week. If you are pregnant and want to enjoy fish caught in local lakes, rivers, or coastal waters, check local advisories about the safety of the fish (and, of course, avoid the fish listed above.) If no advisories are available, it's okay to eat up to 6 ounces of local catch per week, but don't eat any other fish that same week.

Salmon, while packed full of healthy omega-3 fatty acids, can also contain unhealthy levels of certain toxins, according to researchers at Indiana University. In a study that compared wild salmon from the Pacific with farmed salmon, researchers found that the farmed salmon contained much higher levels of PCBs (polychlorinated biphenyls) and dioxins, substances that some studies suggest are linked to cancer. Citing EPA standards for hazardous levels of these chemicals, the study concluded that people should eat no more than 1 or 2 servings of farmed salmon per month. (Wild salmon can be eaten up to 8 times per month, according to the study.)

The American Cancer Society advises that more information about the effect of the toxins is needed, and that the benefits of eating this healthy fish may outweigh the risks. Because many of the toxins are contained in the fat of the fish, you may be able to reduce possible exposure by removing the fatty skin from salmon and broiling or grilling the fish to further reduce its fat content.

Check with your local or state health departments about whether you should be avoiding the local catch. If you are eating fish caught by friends or family in the area, limit it to one meal a week.

-- Mary Roach is a former contributing editor at Health. This article was adapted from a story that first appeared in Hippocrates magazine.

References

Last updated September 22, 2008
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