Article: Contact dermatitis usually shows up when you've been touched by a substance that triggers a reaction, like poison ivy or oak or latex gloves. The rash usually will crop up suddenly, within hours of contact. It may also last for two to four weeks. There are two basic kinds of contact dermatitis, allergic or irritant. Allergic dermatitis occurs when you have contact with an allergenic substance and it produces a delayed reaction. This may develop over a period of 48 to 96 hours, or it may take even longer, between seven and 10 days after the first exposure. Allergenic contact dermatitis can also cause oozing blisters, intense itching, and sometimes swelling in the face, eyes, or genitals. Irritant contact dermatitis occurs when skin comes into contact with an irritant, like a detergent or solvent. Skin damage may be permanent if the irritant is powerful, like acid or lye soap. This kind of dermatitis may also cause mild swelling, itching, blisters, painful ulcers, or tight-feeling and cracked skin. If you note a rash and think you may have come into contact with a plant like poison oak, ivy, or sumac, make sure you wash everything that may have been in contact with the plant with cold water. This includes clothing, gardening tools, sports equipment, and pets. Contact dermatitis happens when a substance comes into contact with the skin your reaction will therefore be limited more or less to where the substance touched you (so if you ran through poison oak with pants on but no shoes, your feet will react but your legs will be spared). If the rash is localized, try to determine if the source is in fact a foreign substance. Is the rash on your hands or face? Contact dermatitis most often affects these areas and rarely occurs on the scalp, palms of hands or soles of the feet. Is the rash confined to the area that was exposed? A delayed reaction sometimes makes it look like a rash from contact dermatitis is spreading, but for the most part it should only occur where your skin was in contact with the irritant or allergen. Contact dermatitis may cause many other symptoms besides redness and a rash. Depending on the strength of the irritant or allergen, you may have pain, blistering, bumps, and severely dry and cracked skin. You may also show non-dermatological signs if the cause is an allergen, like burning lungs, eyes, or nasal passages. Dry skin with a glazed, parched aspect is often the first sign with irritants. Thickened skin and cracking can follow if contact continues for a longer period of time. In very severe cases of exposure to an irritant, the skin may also exhibit burns or tissue death (necrosis). Burning eyes, nose, and lungs can indicate allergic contact dermatitis, but can also point to an irritant in the air.

What is a summary?
Beware of sudden rashes. Check that the rash is localized. Note other swelling, blistering, burning, or tenderness.