U

ultraviolet rays

These rays (also known as UV) have wave lengths shorter than visible light and longer than x-rays. (Ultraviolet means beyond violet—violet being at the top end of visible light’s color spectrum.)

Overexposure to ultraviolet radiation will damage the skin over time and can cause skin cancer, so a sunblock with SPF 15 is advisable. (SPF stands for sun protection factor. An SPF of 15 means that you can spend 15 hours in the sun with—presumably—no more damage than if you spend one hour in the sun without it.) The best protection is provided by UV-absorbers (PABA is a good one) and UV-reflectors (titanium dioxide is a good one).

umbelliferone

A natural phenol found in many plants, such as galbanum or asafetida.

undecylenic acid

Used as an antidandruff agent in shampoos (in a concentration of loo), this organic acid is somewhat similar to the acid in our own perspiration. Nevertheless, it’s a primary skin irritant and a toxic chemical that has murdered lab rats at 2.5 g/kg.

A zinc salt, its chemical name is undecylenic acid monoethanolamidedksodiuIlisulfosuccinate. It’s sold under the trade names of Declid, Renselin and Sevinon.

unsatu rated fats

A fat, usually of vegetable origin, whose fatty acid chains can accommodate more hydrogen atoms. Compare saturated fats.

urea

This end product of normal mammalian metabolism, found in urine and other body fluids, is a white, crystalline or powdery, water-soluble compound. It’s used medically as a diuretic, and is sometimes used in cosmetics. When combined with formaldehyde - urea becomes a syrup. Mixed with cellulose and coloring matter, that syrup is used to treat textiles and in laminating. Many people are allergic to fabrics that contain urea-formaldehyde, and anyone allergic to ammonia will probably be allergic to urea. It’s also dangerous to use products containing urea around the eyes. Because of its high nitrogen content, urea is also used to manufacture fertilizer. Commercially, it’s manufactured by the partial hydrolysis of cyanamide, and by heating carbon dioxide and ammonia under pressure. In 1828, F. Wholer prepared urea from phosgene and ammonia, by rearrangement of ammoniurn cvanate.

uric acid

Sometimes used in cosmetics instead of allantoin or comfrey extracts, which are superior ingredients, this is a nitrogenous compound that’s present in the urine of most mammals. Elevated uric acid is believed to be a symptom of gout.

usnic acid

This antibiotic is used in anti-acne formulas at concentrations of O.1%—0.3% and in deodorants at concentrations of 0.2%. It’s also used as a local therapeutic agent.

Usnic acid is a toxic chemical that’s been used to murder lab rats at 30 mg/kg and mice at 25 mg/kg, and it’s showed subcutaneous toxicity to mice at 700 mg/kg. It’s produced naturally by lichens (Cladonia stellaris, Usnea barbata and others).