Cross-contamination occurs when a food that does not itself contain any allergens is tainted with an allergen during food preparation, cooking, storage, or serving. It can occur at home, in restaurants, or in manufacturing lines.
Cross-contamination is an important food safety concern for people with allergies because many allergies have small threshold levels -- that is, they require only a tiny amount of allergen to trigger an allergic reaction. The amount of allergen protein that can be transmitted to an otherwise safe food through cross-contamination is often beyond this level.
Avoid cross-contamination for "big eight" allergens in packaged foods. The FDA does not require manufacturers to disclose on food labels whether manufacturing lines are used for multiple "major allergens" -- that is, the eight most common food allergens. Many manufacturers voluntarily provide this information on labels, however. A 2001 study showed that between 13 and 22 percent of products with this type of warning did indeed have some detectable amount of contamination.
Avoid cross-contamination from other allergens in packaged foods. For packaged foods that do not include cross-contamination information, for consumers who are allergic to non-"big eight" foods, and for clarification of ambiguous labels like "good manufacturing practices," your best (and sometimes only) recourse is to call or e-mail customer service for clarification. Your tolerance for potential in-factory cross-contamination where you can't get a clear answer should be guided in part by the severity of your potential symptoms, with guidance from your allergist.
Keep your home allergen-free. By far the easiest way to avoid cross-contamination at home is to keep no allergens in the home in the first place. If your doctor has informed you that your allergies carry a high risk of anaphylaxis or other severe reactions, this is the only prudent course of action.
Keep allergens far from safe foods. If you must keep allergens in your home, you can decrease your chances of cross-contamination by keeping them far from food preparation and serving areas, by designating special food prep areas and utensils for allergens, and making sure to clean all food surfaces that have touched allergens thoroughly as soon as you're finished using them. You might also consider using disposable plates and plastic silverware if you serve allergens on only rare occasions.
Avoid the most likely sources of in-home cross-contamination. Some foods are higher risk for cross-contamination than others. It stands to reason that, for example, an allergenic beverage drunk from a can and immediately recycled is far less likely to contaminate safe foods than a crusty, crumbly bread, or a food that is fried in oil and "spitting" all over the place. The most dangerous foods are messy, difficult to clean, or likely to leave crumbs, oil or other traces on surfaces.
Know where cross-contamination occurs in restaurants. Cross-contamination in restaurants is most likely to occur in the actual cooking of food, although cutting boards can be a potential issue. Three major vectors for cross-contamination in restaurants are frying oil, communal grills, and large woks. It is often possible to adapt grilled restaurant dishes to be pan-grilled in separate, clean pans, but this will require some communication with your waitstaff and your chef. Never eat anything fried in oil in which a food you are allergic to has been fried.
Deal with incorrect orders in restaurants. Should you receive an order that is incorrect -- for example, a dinner that was erroneously given to you with an ear of corn, or a breakfast platter that should have been egg-free but isn't -- do make sure, when you send it back, that you specify that you cannot accept the plate simply returned to you with the offending food removed (or replaced). Explain, briefly, the concept of cross-contamination, tell the waiter that the allergen on the plate has made the other foods unsafe for you to eat, and ask (politely) for an entirely new meal.
Beware the salad bar. Salad bars are an especially likely source for cross-contamination because spoons may be moved from one allergen-containing food to another previously safe food. If you are severely allergic to an item on a salad bar, you should generally avoid eating from that salad bar unless precautions to keep serving utensils in their designated containers are scrupulously followed.
Practicing food safety can help minimize cross-contamination risks. Many cross-contamination incidents occur because knives, cutting boards, and spoons were not washed in between different foods. Washing with hot soapy water can prevent this.
Wooden cutting boards are easy on knives but much tougher to get thoroughly clean. Plastic cutting boards are easier to sanitize because they can be run through the dishwasher.
Some foods -- namely nuts and seeds -- can leave oils on plates, tables, and other surfaces. These oils are easily cleaned with most household cleaning agents, but can trigger allergies if not attended to.
Watch what friends have been eating and drinking before letting them drink from your cup. Hands and faces are a potential vector for cross-contamination too.
Be wary of bagel cutters and meat slicers, and similar items if you have allergies that would be triggered by foods that may have been cut on these. Similarly, bulk bins can be a source of cross-contamination if scoops are transferred between items, though many stores use attached scoops or hopper-style bins to minimize these risks. When in doubt, ask a manager about cross-contamination precautions.