Woman lying on the beach wearing fedora looking at the ocean
First lets start with:

WHAT
Ultraviolet (UV) radiation comes from natural sources (like the sun), and artificial sources (like black lights, welding equipment, lasers, and tanning equipment).
There are different types of radiation, depending on its wavelength, different types of solar radiation do different things:
Infrared radiation makes us feel the warmth of the sun.
Visible light allows us to see the world around us.
Ultraviolet radiation affects our health.
Ultraviolet radiation is invisible energy in the wavelength range from 100 to 400 nanometers (nm). A nanometer is one billionth of a meter. UV radiation has a shorter wavelength and is more energetic than visible light. Depending on its wavelength, it can get through the ozone layer and affect our health in different ways. The shorter the wavelength, the more harmful the UV radiation. However, shorter wavelength UV radiation is less able to penetrate the skin.
UV radiation is divided into three wavelength ranges:
UVA is long-range UV radiation between 320 and 400nm. Although not as energetic as UVB, UVA can penetrate deep into our skin (dermis). This can cause immediate tanning and premature skin aging, and play a role in the development of certain skin cancers. UVA is not readily absorbed by the ozone layer - about 95% gets through.
UVB is short-wave UV radiation between 280 and 320nm. It can just penetrate the outer protective layer of the skin and is responsible for delayed tanning, sunburns and most skin cancers. A large amount of UVB is absorbed by the ozone layer - only 5% reaches our planet's surface.
UVC, with wavelengths between 100 and 280nm, is very energetic. It is very dangerous to all forms of life (even with short exposures). However, UVC radiation is filtered out by the ozone layer, and never reaches earth. It is created artificially to kill bacteria.

People on the beach on a hot sunny day
WHY
As you can see UV radiation can be a concern to our health in many different ways, by protecting you from harmful radiation from the sun, sunscreen reduces your risk for skin cancer, premature aging and sunburns. Your skin can be harmed by constant sun exposure, whether or not you see a burn. Remember, sunburn is an immediate reaction, but damage from the sun occurs over a lifetime. You can optimize your protection by choosing the right sunscreen, wearing it regularly and using it as directed.

There are different types of SPF to choose from, before sunscreens were known for protecting the skin from UVB Rays, which rates how well the sunscreen protects against one type of cancer-causing UV ray, ultraviolet B. However, research soon showed that ultraviolet A rays (UVA) also increase skin cancer risk. While UVA rays don't cause sunburn, they penetrate deeply into skin and cause wrinkles. The Environmental Protection Agency estimates that up to 90% of skin changes associated with aging are really caused by a lifetime's exposure to UVA rays.
You'll want to use and SPF 15 or higher to protect your skin from UVB rays. There is no rating to tell you how good a sunscreen is at blocking UVA rays, so when it comes to UVA protection, you need to pay attention to the ingredients.

SPF 30 is the most common level for most people and skin types. SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97% of UVB rays and SPF 50 blocks 98% of UVB rays. So, the difference between 30 and 50 is about 1 percent. For protection against UVA rays look for sunscreen that contains one of the following: ecamsule, avobenzone, oxybenzone, titanium dioxide, sulisobenzone, or zinc oxide.
Woman applying skin cream on her face looking in the mirror
WHEN
UV radiation from the sun reaches its peak at solar noon, which is between 12 and 1 p.m. across Canada. At this time, the sun's rays have the least distance to travel through the atmosphere, increasing the intensity of UV. In general the UV Index in Canada can be 3 or higher from 11 a.m. to 3 p.m.
Sunscreens should be applied 30 minutes before sun exposure to allow the ingredients to fully bind to the skin. Re application of sunscreen is just as important as putting it on in the first place, so reapply the same amount every two hours.
During the year, the sun's angle varies, which causes the intensity of UV rays to vary. UV intensity is highest during the spring and summer months. But the sun can still have an effect on your skin and eyes in the fall and winter, especially when UV is reflected back by large surfaces of fresh snow. So using sunscreen all year round is beneficial whether you're living in a hot climate or experiencing cold winter months.
Thanks for stopping by I hope this blog answered some of your questions related to sunscreen and why it's important. If there's any topic you would like me to write about please leave your comments below.  Enjoy your summer, remember to protect your skin! :)

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