A pH level, scientifically speaking is the measurement of hydrogen ions in a substance. The purpose of this measurement is to read the acidity or alkalinity of a particular substance. This is an important evaluation, especially for cosmetics, because the pH level if very high or very low can be dangerous if it comes into contact with another substance, such as skin.

The pH scale goes from zero to 14. Zero being very acidic and 14 being very alkaline, with everything in various spots in between. Lemon juice is registered at a pH level of two, vinegar at two and a half and peroxide at four. These are all acidic. At the other end of the scale depilatories (hair dissolving agents such as Veet and Nair) come in at 11.25, soap registers at ten and pure water is smack in the middle at seven.

Skin varies, depending on the person and often their diet, between 4.5 and 6. The norm is 5.5, which is slightly acidic. There are theories that the reason for skin's slight acidity is to fight bacteria and keep the skin healthy. As we age, the pH of the skin begins to slide to a more neutral level, lessening the acidity.

This information is important if your products are causing your skin to be dehydrated and sensitive. Often, using regular soap will break down the skin's acidic defenses because of its alkaline nature. Regular soap normally has a pH of 10, whereas skin is well on the other side of neutral at 5.5. This is why using regular soap will leave your skin feeling tight and dry.

Cosmetics should be balanced for the skin, which would make them slightly acidic. If you're using a treatment cleanser for acne or oily skin often it will have a more alkaline reading than a cleanser for normal or dry skin. This is to help treat the oil secretions.

If this is the case, it's important to follow your cleanser with a toner to return your skin to its normal pH level. You still have the benefit of a strong anti-sebaceous cleanser without the repercussions of an unbalanced skin, which is dehydration.

Even if you use a cleanser for normal or dry skin, it's a good idea to follow up with a toner because the water you rinse with after cleansing still has a higher pH than your skin.

Ensuring your products are pH balance can be difficult if the manufacturer's label doesn't inform you. This is usually the case with cheaper cosmetics sold off the shelf in drug or grocery stores. Spa and salon cosmetics, as well as department store cosmetics will have this information more easily.

Manufacturers of the more expensive lines of cosmetics will almost always ensure the pH level of their products are specifically designed for skin.

If you're concerned about your specific cosmetics ask your esthetician or sales associate about the products. You may find a toner is all you need to balance your skin.

Denise Surette loves toners with chamomile and lavender extracts to calm and soothe the skin. She is a journalist and an esthetician at HS Studio.