Understanding the active ingredients in sunscreen is a bit like learning a foreign language that uses a different alphabet. Suffice it to say in short that sunscreens are made up of a combination of two types of ingredients: those that reflect ultraviolet (UV) rays and those that absorb UV rays. Although that might sound simple enough, there are also two types of rays: UVA rays and UVB rays. UVB causes sunburn at the outer layers of skin, but UVA causes sun damage that reach deeper layers of skin. Both UVA and UVB radiation can contribute to the development of skin cancer.
Homosalate (homomethyl salicylate; HMS) is a UVB ray filter used in many sunscreens. It does not protect against UVA. It is an oil-soluble chemical which degrades when exposed to sunlight. Like all salicylates, it is not powerful enough to stand on its own in UVB protection and is almost always combined with other filters.
Many countries have placed limits on its concentration – 15% in the US and Australia and only 10% in Japan and Europe.
Keep in mind that a sunscreen’s Sun Protection Factor (SPF) is a gauge of how well the formula protects the skin from UVB rays. It does not gauge protection from UVA rays.