There are two different ways sunscreens protect you from UV radiation.
The first type is a physical sunscreen and the second is a chemical sunscreen. The first acts as a physical block, which helps in blocking the the UV light from reaching the skin. The product reflects light away from the skin by the use of two main ingredients which are zinc oxide or titanium dioxide. The biggest complaint people have are that these types are not esthetically pleasing because of their opaque white paste. In the last few years manufacturers have developed products that blend better into the skin while still maintaining their reflective abilities.
The second type of sunscreen is chemical and works to absorb the UV radiation hitting your skin and chemically convert it into infrared. Unlike the physical block a chemical sunscreen requires active absorption of the radiation. The active ingredients must be absorbed into the skin to provide protection. These products tend to work better on UVB than UVA rays and also tend to lose their effectiveness faster than the physical type and require more frequent re-application. Finally, unlike a physical sunscreen they may tend to be more allergenic.
All sunscreen ingredients provide protection over some part of the UV spectrum. Some include both UVA and UVB while others are only for UVB. UVA is not as easily absorbed by chemical sunscreens so physical sunscreens provide a broader protection and may be more appropriate for our strong New Mexico sun.
Water and Non-Waterproof
This classification is based on the product being able to withstand 40 or 80 minutes of immersion without losing its effectiveness. In the US a product that meets the 40 minute standard can be labeled water resistant. A product that can meet the 80 minute standard can be labeled waterproof. The longer a product will stay on the skin, the less likely it will have a “lighter” feel. It will generally feel thicker and oilier on the skin.