The two terms are sometimes used interchangeably, sunscreen and suntan lotion. But, evidenced by the meanings of their names, these products do very different things. As the names suggest, sunscreen screens the sun's rays from the skin and to prevent burns and tanning, which damage the skin. Conversely, suntan lotion is a broader term that references topical lotions that help the skin to tan, or that do not have any protective ingredients.
Sunscreen is a term to designate lotions that contain ingredients that work to prevent damage to the skin from the sun. These topical sunscreens contain ingredients that reflect the sun's ultraviolet rays from the skin. Sunscreens are often accompanied by an SPF level of 5 through 45 or higher to indicate the level of protection.
Sunscreens are lotions containing aromatic molecules conjugated with carbonyl groups. These function to absorb the high energy of the UV rays, then reflect them as lower energy UV rays before the rays have a chance to reach the skin. This prevents burn, and over time, can prevent carcinoma or skin cancer, according to the American Cancer Society.
Products designated as "suntan lotion" should definitely not be confused for protective products such as sunscreen or sunblock. Suntan lotion does not contain any of the protective ingredients of sunscreens and sunblocks, and in many cases is actually used to aid tanning instead of prevent damage from the sun.
Suntan Lotion Ingredients
The ingredients in suntan lotion increase the effects of ultra violet rays (instead of blocking them) by aiding the production of melanin, or by increasing blood flow to the surface of the skin. Some suntan lotions contain melanin as an ingredient. Other ingredients include tea oil, green tea oil and copper. Many lotions claim to be moisturizing, but this often only means that they do not contain alcohol, which can dry out the skin.
How to Tell the Difference
Because many lotions use different terms to describe their function, sun tan lotion, tanning oil, sunscreen and sun block, here are some helpful indicators. If a product has the term "tan" in it, it is likely a tanning aid that will not protect your skin from the sun's ultra violet rays. If the product mentions an SPF, or Skin Protection Factor, and a number, it is a topical agent that will protect your skin from burning. Additionally, the Skin Cancer Foundation often displays its seal on approved protective sunscreens and sunblocks, which might help you make your choice.