What Is Urea, What Does It Do? Effect on the Skin by Concentration

Urea is known as the molecule that carries the body's nitrogen waste, but it has another job in the skin. It is one of the components of the natural moisturising factor (NMF) in the upper layer and helps keep water there. Urea used in cosmetics is not obtained from urine; it is synthesised from ammonia and carbon dioxide. That a single molecule does a different job as its proportion changes is what makes it interesting.

How does its effect change when the concentration changes?

For urea, the percentage written on the product tells almost as much as the product type.

These ranges are the general framework in reviews; there is no single standard. Keratolytic effect means loosening of the bonds that hold dead cells together. High-concentration products are not made to be applied to the face as a daily moisturiser. Since urea can also increase skin permeability, it may facilitate the passage of other ingredients applied with it. If you plan to use it alongside prescription creams, tell your doctor.

Is the barrier effect limited to moisture?

Urea was long regarded as a passive humectant. A placebo-controlled study in 21 healthy volunteers suggests more. With 20% urea used once a day for 4 weeks, transepidermal water loss fell by 31% (from 10,0 grams to 6,9 grams per square metre per hour), while 10% urea did not change this measurement significantly. In biopsies, barrier proteins, enzymes involved in fat synthesis and some antimicrobial peptides increased.

A human study was supported by a company that produces urea products, and some of the researchers have ties to that company. The result should be read as a biological clue, not as treatment of a disease such as eczema. A study of 172 people with atopic dermatitis also reports that 5% urea reduced the risk of relapse by 37%, but decisions in the disease picture rest with the doctor.

Why is it often recommended for feet and very dry skin?

The skin of the sole is thick and has no sebaceous glands, so the keratolytic effect works well there. In a systematic review on dry feet, the most studied group was urea creams: studies at 5-40% (14 studies) reported good clinical response. In a study of 40 people with diabetes, 5% urea together with arginine and carnosine corrected moisture and dryness better than a glycerin cream. Also in diabetes, in a pilot study of 30 people, 25% urea outperformed 10%.

When you read these data, also keep in mind the review authors' connection with urea product manufacturers. For steps against thickening and cracks on the heels, see the heel cracks article. If you have diabetes and notice a wound, a bleeding crack or numbness on your foot, do not apply strong keratolytics on your own; see a doctor.

Why can it sting on sensitive skin?

The best-known problem with urea products is mild stinging and itching. In a double-blind study of 197 atopic patients, a cream containing 4% urea and 4% salt caused more stinging than a 20% glycerin cream. Since the cream also contains salt, it would not be correct to put the responsibility on urea alone. Dryness assessments were similar between the groups. Overall tolerance is good, side effects are mild, and they occur more often at high concentrations.

• Open and bleeding skin: Do not apply to bleeding, open, infected or weeping cracks. Do not use on widespread redness either.
• Children: For diagnosed dryness, use under a doctor's advice from age 1. For normal dry skin without a diagnosis, the lower age limit for low-concentration products is usually 3.
• New product: If your skin is sensitive, first apply it to the inner arm or elbow crease, twice a day, for 7-10 days. If spreading redness or a rash appears rather than transient stinging, stop.
• Smell: Some creams may have a smell from volatile amines that disappears quickly.

Not every substance with "urea" in its name is urea

If you see Imidazolidinyl Urea or Diazolidinyl Urea on the ingredient list, these are not moisturising urea; they are formaldehyde-releasing substances used as preservatives. Hydroxyethyl Urea is a separate humectant derived from urea. If you are looking for urea itself, the list should say only "Urea". For the whole class of humectant ingredients, see the humectant page; for the ingredient most often compared with urea, see the glycerin page.