The science of sunscreen
Why does sunshine cause sunburn and skin cancer, and how does sunscreen help to protect you? Jake Port explains.
When we think of life saving inventions, vaccines, antibiotics and other medical breakthroughs come to mind. Often missing from this list is one of the most common tools for cancer prevention: sunscreen, the amazing lotion that can stop harmful radiation in its tracks.
Before delving into how sunscreen works, we need to first understand the injury that we are trying to prevent in the first place, sunburn.
We generally know that we have sunburn because of the change in skin colour that can appear soon after spending time in the sun.
The colour change is a result of inflammation, the immune response generally associated with bruising or infection. With sunburn, the cause of the inflammation is deliberate cell death, known as apoptosis. Cells are intentionally killing themselves to prevent potentially cancerous mutations occurring.
But why would mutations be occurring in the first place? Radiation emitted by the sun. This radiation pummels the skin, entering the cells and causing damage to the delicate DNA residing inside the nucleus.
Not all radiation can reach this far into the cells: the kind you need to worry about is ultraviolet (UV) radiation, which sits next to visible light on the electromagnetic spectrum.
UV light comes in three main forms. UVA has the lowest energy (longer wavelength) but highest penetrative ability, UVC has the most energy (shorter wavelength) and lowest penetrative ability, and UVB sits in the middle.
In terms of sunburn, UVC isn’t relevant as the radiation is absorbed in the upper atmosphere. UVB on the other hand present more of danger, it has a dangerous combination of penetrative ability and energy that can cause DNA damage. UVA can be hazardous but mostly it causes the tanning effect that beachgoers often crave.
When UV radiation reaches our skin, specialised cells known as melanocytes produce melanin, the photoprotective pigment that causes skin to darken or tan. After a certain point these cells are overwhelmed and radiation starts to penetrate both skin cells and melanocytes, causing apoptosis (deliberate death). Later this skin, full of cells that have died to protect the body, will peel off revealing a new skin layer beneath.
Melanocytes are more vulnerable to becoming cancer cells than other cell types as they have a special ability to prevent normal apoptosis, allowing them to live for decades. This ability is a double-edged sword: if it mutates it can turn a long-lived melanocyte into a cancerous melanoma. The deadliest type of skin cancer, melanoma occurs when melanocytes divide uncontrollably, producing a distinctive misshapen mole that appears at the skin’s surface.
Luckily, the chance of sunburn and skin cancer can be massively reduced by applying a simple layer of sunscreen.
Applied as a lotion, spray or wax, sunscreen can consist of organic and inorganic compounds that chemically and physically protect the underlying skin.
Inorganic compounds, such as zinc oxide or titanium dioxide, reflect harmful UVB and UVA in the same way that white paint reflects light off a surface.
Organic molecules including benzophenone and cinnamates absorb the energy of the UV radiation and convert it to heat. While they lack the often-white appearance of physical blockers, chemical blockers don’t last as long in the sun, requiring them to be re-applied more often.
While sunscreen can be a big help, to keep your skin safe remember to avoid unnecessary sun exposure and use sunscreen correctly: put it on whenever you’re out in the sun, and re-apply every two hours.