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Why sunburns happen on cloudy days…and more!

What is the buzz about UVA these days?

We now understand that UVA (in addition to UVB) contributes to skin cancer development.  On a summer day, UVA accounts for >95% of the UV light reaching the earth’s surface, while UVB accounts for just 4%.  The ozone layer and atmosphere block essentially all UVC light emitted by the sun and nearly 90% of UVB light, but they block hardly any UVA or visible light.

What if I do not go outside during the peak hours of sunlight?

UVB light is strongest between the peak hours of 10am to 2pm, when the sunlight travels the most direct path through the atmosphere. However, the intensity of UVA light is relatively constant throughout the day, regardless of the hour.  UVA light is also minimally affected by cloud cover.

Why can I still get sunburn on a very cloudy day?

Cloud cover reduces the intensity of UV light, visible light, and infrared radiation (IR).  IR produces the sense of warmth from the sun.   Clouds reduce IR transmission more than UV transmission.   Without the warning sensation of warmth, people are at increased risk of overexposure to UV light, especially UVA which is relatively unaffected by cloud cover.  The US National Weather Service’s calculation of the UV index assumes that clear skies allow virtually 100% of UV transmission, scattered clouds 89%, broken clouds 73%, and overcast skies 31%.  Even on days when you cannot see the sun, you are still getting significant UV exposure.  Apply your sunscreen every day before leaving the house.

How protected am I in the shade or under an umbrella?

Small shade structures like an umbrella provide only low UV light protection.  Single trees provide an SPF (Sun Protection Factor) from 4 to >50, depending on the foliage density and proximity to the shadow perimeter.

Why do I still need to wear sunscreen if I am always indoors or in a car?

Although Americans spend much time in vehicles or indoors, the potential for UV exposure through car and building windows is often overlooked.  In the US, 2 recent studies demonstrated a significant increase in skin cancers on the left side of the body, i.e. the driver’s exposed side.  Standard residential and commercial window glass blocks the transmission of UVB but not UVA (think how the letter A passes through the middle of the word glAss).  Newer types of glass can partially block UVA and visible light, depending on glass thickness, type, and color.  To promote safety, all automobile window shields are made of laminated glass, which filters UVB and most UVA.  However, the side, rear, and overhead windows are usually made of tempered glass that does not as effectively block UVA.  Tinted glass reduces more UVA penetration than non-tinted glass.

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