RALEIGH — With all the warnings about protecting our skin, you would think that everyone would be using so much sunscreen that we would never have to worry about it getting old too old to use.

If you are not sure long you have had a bottle of sunblock, it is time to find out. Look for the expiration date on the bottle or tube.

The date give you two years to use the sunscreen and get its maximum protection.

Over time, the protective ingredients in sunscreen can break down, making skin fair game for the sun's blistering rays.

Old sunscreen can also trigger an allergic reaction.

"We have seen some individuals who seem very irritated or allergic to some of the ingredients in sunscreen that clearly they could tolerate in the past," says dermatologist Dr. Robert Johnson.

Not all bottles have an expiration date. If tests show the product remains stable for at least three years, the FDA does not require one.

If you do not know how long you have had the same bottle of sunscreen, doctors recommend using common sense -- and your senses -- as a guide.

"If it doesn't smell very good, if it feels watery and not the same texture, toss it out," says Johnson.

Another problem in protecting skin is that most of us only use about half of what we need to. The rule of thumb for an average sized person is one ounce of sunscreen per application.