United States 

Environmental Protection

Agency 

Air and Radiation 

6205J 

 

EPA 430-F-10-025 

June 2010 

www.epa.gov/ozone/strathome.html 

UV Radiation 

This fact sheet explains the types of ultraviolet radiation and the various factors that can affect 

the levels reaching the Earth’s surface. The sun emits energy over a broad spectrum of 

wavelengths: visible light that you see, infrared radiation that you feel as heat, and ultraviolet 

(UV) radiation that you can’t see or feel. UV radiation has a shorter wavelength and higher 

energy than visible light. It affects human health both positively and negatively. Short 

exposure to UVB radiation generates vitamin D, but can also lead to sunburn depending on an 

individual’s skin type. Fortunately for life on Earth, our atmosphere’s stratospheric ozone layer 

shields us from most UV radiation. What does get through the ozone layer, however, can 

cause the following problems, particularly for people who spend unprotected time outdoors: 

● Skin cancer 

● Cataracts 

● 

● 

 

 

Suppression of the immune system 

Premature aging of the skin 

Did You 

Know? 

Ultraviolet (UV) 

radiation, from the 

sun and from 

tanning beds, is 

classified as a 

human carcinogen, 

according to the 

U.S. Department 

of Health and 

Human Services 

and the World 

Health 

Organization. 

Since the benefits of sunlight cannot be separated from its damaging effects, it is important to 

understand the risks of overexposure, and take simple precautions to protect yourself. 

Types of UV Radiation 

Scientists classify UV radiation into three types or bands—UVA, UVB, and UVC. The ozone 

layer absorbs some, but not all, of these types of UV radiation: 

● 	 UVA: Wavelength: 320-400 nm. Not absorbed by the ozone layer. 

● 	 UVB: Wavelength: 290-320 nm. Mostly absorbed by the ozone layer, but some does reach 

the Earth’s surface. 

● 	 UVC: Wavelength: 100-290 nm. Completely absorbed by the ozone layer and 

atmosphere. 

UVA and UVB radiation that reaches the Earth’s surface contributes to the serious health 

effects listed above; it also contributes to environmental impacts. Levels of UVA radiation are 

more constant than UVB, reaching the Earth’s surface without variations due to the time of day 

or year. In addition, UVA radiation is not filtered by glass. 

UV Levels Depend on a Number of Factors 

The level of UV radiation reaching the Earth’s surface can vary. Each of the following factors 

can increase your risk of UV radiation overexposure and consequent health effects. 

Stratospheric Ozone Layer 

The amount of UV rays the ozone layer absorbs varies depending on the time of year and other 

natural events. Additionally, the ozone layer is thinner than it used to be due to ozonedepleting chemicals used in industry and consumer products. These chemicals are being 

phased out, but the ozone layer is not predicted to heal to pre-1980 levels until mid– to latecentury. 

Time of Day 

The sun is highest in the sky around noon. At this time, the sun’s rays have the least distance 

to travel through the atmosphere and UVB levels are at their highest. In the early morning and 

late afternoon, the sun’s rays pass through the atmosphere at an angle and their intensity is 

greatly reduced. 
Time of Year 

The sun’s angle varies with the seasons, causing the intensity of UV 

rays to change. UV intensity tends to be highest in the summer. 

Latitude 

The sun’s rays are strongest at the equator, where the sun is most 

directly overhead and UV rays must travel the least distance through 

the atmosphere. Ozone also is naturally thinner in the tropics 

compared to the mid- and high-latitudes, so there is less ozone to 

absorb the UV radiation as it passes through the atmosphere. At 

higher latitudes, the sun is lower in the sky, so UV rays must travel a 

greater distance through ozone-rich portions of the atmosphere and, 

in turn, expose those latitudes to less UV radiation. 

Altitude 

UV intensity increases with altitude because there is less atmosphere 

to absorb the damaging rays. As a result, your chance of damaging 

your eyes and skin increases at higher altitudes. 

Weather Conditions 

Cloud cover reduces UV levels, but not completely. Depending on the 

thickness of the cloud cover, it is possible to burn on a cloudy day, 

even if it does not feel warm. 

Reflection 

Surfaces like snow, sand, pavement, and water reflect much of the UV 

radiation that reaches them. Because of this reflection, UV intensity 

can be deceptively high even in shaded areas. 

EPA’s SunWise Program 

In response to the serious public health threat posed by exposure to 

UV rays, EPA works with schools and communities across the nation 

through the SunWise Program. SunWise teaches children how to 

protect themselves from overexposure to the sun. For more 

information, please visit www.epa.gov/sunwise. 

The UV Index 

The UV Index forecasts the strength of the sun’s harmful rays. 

The higher the number, the greater the chance of sun damage. 

Visit www.epa.gov/sunwise/uvindex.html 

Be SunWise 

● Do Not Burn 

● Avoid Sun Tanning and 

Tanning Beds 

● Use Sunscreen 

● Cover Up 

● Seek Shade 

● Watch for the UV Index 

● Use Extra Caution Near 

Water, Snow, and Sand 

● Get Vitamin D Safely 

UVC

UVB

UVB

EARTH

UVA

The stratospheric ozone layer 

screens out much of the sun’s 

harmful UV rays.