Chemical burns are tissue damage caused by strong acids, drain cleaners, paint thinner, gasoline and many other substances. Usually, you are aware of such a burn and its cause. But sometimes you may not immediately recognize a burn caused by a milder chemical. As with some sunburns, the damage may develop hours after the exposure.

Major chemical burns need emergency medical help. Minor chemical burns can usually be treated with first aid.

Call 911 or seek immediate care for major chemical burns, which:

• May be deep, involving all layers of the skin.
• Are larger than 3 inches (about 8 centimeters) in diameter.
• Cover the hands, feet, face, groin, buttocks, or a major joint or encircles an arm or leg.

If you're unsure that you've been exposed to a toxic chemical, call a poison control center then call 911. In the United States, the Poison Help number is 800-222-1222. If you seek emergency medical help, take the container or the name of the chemical with you.

For major chemical burns, apply first aid as follows until emergency help arrives. For minor burns, take the same steps. A minor burn might need emergency care if it affects the eyes, mouth, hands or genital areas. Babies and older adults might need emergency care for minor burns as well.

• Protect the burned person from further harm. Remove dry chemicals. Put on gloves and brush off any remaining material.
• Remove contaminated clothing or jewelry and rinse chemicals off for at least 20 minutes, in a shower if it's available. Protect eyes from the chemicals.
• Cover the burn. Loosely cover the area with gauze or a clean cloth.
• Rinse again if needed. If the area is still painful, rinse for several more minutes.

For major burns, watch for signs of shock. Symptoms include cool, clammy skin, weak pulse and shallow breathing.