Ultraviolet Radiation

UVR is part of the electromagnetic spectrum emitted by the sun and artificial sources, such as tanning beds. UVR cannot be seen or felt – it’s not related to visible light or to temperature.

The UV spectrum can be subdivided into three types: UVA, UVB, and UVC.

Only UVA and UVB radiation reach the earth’s surface. UVB radiation is generally associated with sunburns though both types of UVR can cause skin damage, skin aging and skin cancer.

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UVR can reach people on the ground these ways:

Directly, from the sun

Sun height: Maximum UVR levels occur when the sun is at its highest point in the sky (solar noon) during the summer months. Most of the daily total UVR is received within 2 hours before and after of solar noon. This is why most sun safety communications encourage people to take extra precautions or to avoid outdoor activities between 11:00 am and 3:00 pm (the Critical Protection Period) from April to September.


Indirectly, scattered by clouds or other particles in the atmosphere

Cloud cover: UVR levels are highest under cloudless skies, but UVR levels can remain high especially during thin or scattered cloud cover. Some clouds can actually increase the UVR intensity on the ground by reflecting the sun’s rays back towards earth.


Indirectly, reflected from surfaces

Ground reflection: UVR is reflected or scattered to varying extents by different surfaces. For example, snow can reflect as much as 80 % of UVR, dry beach sand about 15%, and sea foam about 25%.

Critical Protection Period

UV radiation from the sun is most intense during the midday hours of 11 am to 3 pm between April and September in Canada (based on average UV Index values measured by Environment Canada).

Scheduling outdoor activities earlier or later in the day can help reduce UVR exposure.

UVR is also more intense during late spring and early summer, at higher altitudes, places closer to the equator, and when reflected off surfaces such as snow, water, and sand.

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  • The Surgeon General’s Call to Action to Prevent Skin Cancer. Washington, DC: U.S. Dept of Health and Human Services, Office of the Surgeon General; 2014.
  • http://www.sunsmart.com.au/uv-sun-protection/uv
  • The Recommended Core Content for Sun Safety Messages in Canada. Briefing on the Results of the 2014/15 National Consensus Process.
  • Figure adapted with permission from Guidelines to Shade, Cancer Council NSW, Sydney, 2013.

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