What do UV Rays do to Your Body?
UV (ultraviolet) light has a smaller wavelength than the visible light spectrum, in the range of 100-400 nanometers (that's 10-9 meters), and contains a lot of energy. There are three types of UV rays: UVA (long wave), UVB (mid-wave), and UVC (short wave). UVA and UVB are the ones we are most often exposed to, which can cause DNA damage, sunburn (technical term: erythema), photoaging, immunosuppression, and cancer. UV initiates a cascade of events in the skin, starting with the absorption of UV rays by chromophores. This "excites" the chomophores, which can react with other molecules and form free radicals and Reactive Oxygen Species (ROS). These are highly reactive molecules, which can interfere with DNA and cause DNA damage. Free radicals and ROS can move freely in the body, which may explain why some melanomas are often found in parts of the body that don't get a lot of sun exposure. This is known as indirect DNA damage. When UV radiation directly interferes with DNA, it is called direct DNA damage. Direct DNA damage creates bonding of thymine nucleotides, which disrupts the strand and prevents DNA replication. When this happens, it triggers apoptosis.
Responding to DNA Damage
When your body senses UV-induced DNA damage, it triggers a signal cascade, which acts like a relay race in the cell that passes along the message and activates all kinds of responses. These responses include the induction of DNA repair mechanisms and the production of melanin. This is a simplification of the signalling response to UV radiation:
Melanin is a skin pigment that absorbs UV light - diverting it away from those pesky chromophores, and protecting your cells from DNA damage. That's the reason why people say they need to build a base tan before going on vacation. People with darker skin naturally have more melanin, while us pale folks have less. Having less melanin means that your skin is less protected from UV rays, which can cause sunburn. Tanning is your skin's response to sun damage.
When DNA is damaged, a checkpoint protein, known as tumor protein p53, is expressed. This protein regulates the cell cycle and functions as a tumor suppressor. Nucleotide excision repair pathways are also induced, as is the alteration of microRNA expression (these are gene silencers). The diagram at the right shows an example of a DNA repair pathway:
When these DNA repair pathways shut down or become impaired, as they do with age, it leads to immune suppression, inflammation, and cancer.
There are two types of sunblock: physical (they reflect sunlight) and chemical (they absorb sunlight). Some sunblocks only protect against UVB rays, which is why health care professionals recommend using a broad-spectrum UVA/UVB sunblock. There has been some controversy over whether or not sunblocks actually prevent melanomas, but authors of these studies often point to the fact that wearing sunblock may encourage people to participate in risky behaviours, such as laying out in the sun longer, and that people often do not use adequate amounts of sunblock. On the other hand, seeking shade and wearing protective clothing was associated with lower melanoma risk, but that's likely because people who participate in these behaviours are more conscious about their sun exposure risk.
But what about vitamin D? Vitamin D is the "sunlight vitamin". It is produced when UVB photons are absorbed in the skin and photolyze 7-dehydrocholesterol, forming previtamin D3, which is isomerized by the body's temperature into vitamin D. Most people are dependent on sunlight to obtain adequate amounts of vitamin D, and it is estimated that 50% of the global population is vitamin D deficient. BUT you can get enough sunlight to produce adequate amounts of vitamin D through casual sun exposure, like during your lunch break. Laying out in the sun for hours is excessive sun exposure, and is the more damaging equivalent of taking multi-vitamins.
Sun protection is important, especially for children. Skin cancers are on the rise, and this is a huge public health issue. But just like anything else that has to do with health, there is a lot of pseudoscience and misinformation out there. There is no consensus on sunblocks causing cancer, and it is largely believed that they are more beneficial than harmful. The important thing to remember is that there is not a single health organization that does not recommend the use of sunblock.