Doctors sometimes test for allergy to specific chemicals or compounds by placing a dilute amount of the substance in question on the skin and observing any reaction. But because some products contain such a large number of chemicals and compounds, it's not possible or practical to test every single one.

So doctors find it easier simply to apply a sample of the cloth, cosmetic or other suspected cause of allergy to the skin. If there is no reaction, the result is considered negative. If there is redness or swelling, it's possible and the material must be avoided. This is called a "patch test." And you can do your own.

To test cosmetics, place a small amount of the suspected cosmetic to the more sensitive skin of the inner forearm, twice daily. If you are truly allergic, you'll react at the site within one or four days. You can patch test up to four cosmetics at a time, one on each forearm and one on the back of each knee.

To test sunscreens, rub a small amount on your forearm (protect the rest of your body with clothing), and spend about 15 minutes in the sun. If you react, test again a few days later with another sunscreen until you find one you can use safely. To test cloth, snip a small sample from an inside seam, corner, or other unobtrusive spot (when buying through mail order, ask for sample swatches). Affix if to be the forearm with a non-allergenic tape.

Do not self-test for industrial-strength chemicals. You could get a very nasty chemical burn. It's also important that you test only when your skin has completely healed from an allergic encounter. If you patch test while you are reacting, you not only run the risk of aggravating the existing problem, but you will be more likely to get a false reading from the rest.