Skip to main content
Hong Kong Observatory Brand Hong Kong - Asia's world city
GovHK Traditional Chinese Simplified Chinese Search Search Site Map Contact Us
Print Version
Back
Print Version PDF Version
UV Radiation Information

Health effect and protective measures against UV radiation

Information on UVA radiation


What is UVA radiation?
Effect of UVA radiation on health
The characteristics of UVA radiation
Protection against UVA radiation
Today's UVA radiation intensity


 
What is UVA radiation?

UV radiation can be classified according to their wavelength (in unit of nanometre, nm) as UVA (315-400nm), UVB (280-315nm) and UVC (100-280nm). UV radiation reaching the earth's surface consists of UVA and UVB only, as all UVC is absorbed by the atmosphere. Over 98% of the UV radiation reaching the ground is UVA and less than 2% is UVB.



 
Effect of UVA radiation on health

Excessive exposure to UVB may cause sunburn and is one of the major causes of skin cancer. In the past, the harmful effect of UVA is often neglected by people as it is much less powerful to induce sunburn than UVB. Recent studies showed that over-exposure to UVA may lead to darkening, aging and wrinkling of the skin, as well as potential development of skin cancer.  Both sunburn and tanning are signs of skin damage.



 
The characteristics of UVA radiation

Since the wavelength of UVA is longer than that of UVB, UVA is less scattered by air molecules as compared to UVB. As a result, the variation of UVA is less than that of UVB during the same time interval. This phenomenon is more prominent during the few hours after sunrise and before sunset when the sun elevation is low. For the same reason, the difference in UVA intensity between summer and winter is smaller.



 
Protection against UVA radiation

Protective measures against UVA include wearing appropriate clothing, wearing sunglasses, using an umbrella and applying sunscreen lotion.

At present, the SPF (Sun Protection Factor) is an international standard for measuring the degree of protection against UVB offered by a sunscreen lotion.  Although there is no international consensus on the measurement of UVA protection by sunscreen lotion at the moment, the PA (Protection Grade of UVA) is commonly used in Asia. There are 3 PA grades: PA+, PA++, and PA+++, with each additional plus (+) indicating a higher protection.  When buying a sunscreen lotion, people should note the SPF as well as the PA and choose one that can block both UVA and UVB.

The UV index and the corresponding UVA intensity, as well as the protective measures, are shown in the table below.

 

 

 

UV Index Exposure level Corresponding UVA intensity*(W/m2) Recommended protective measures

0-2

Low

0-20

 

No special protective measures required.

3-5

moderate

21-35

  • Seek shade during midday hours when staying outdoors
  • Use a sunscreen lotion that can block both UVA and UVB (with SPF 15 and PA+, or above)
  • Wear long-sleeved and loose-fitting clothing, wear a broad brim hat, use an umbrella, wear sunglasses

6-7

High

36-45

8-10

Very high

46-55

  • Avoid being outdoors in the sun during midday hours
  • If you must stay outdoors, seek shade
  • Use a sunscreen lotion that can block both UVA and UVB (with SPF 15 and PA+, or above)
  • Wear long-sleeved and loose-fitting clothing, wear a broad brim hat, use an umbrella, wear sunglasses

>=11

Extreme

>=56

*The reference values are obtained from an analysis of UVA radiation data collected in Hong Kong by the Observatory during 2010. There exists a certain degree of correlation between the UVA intensity and UV index in Hong Kong.



 

 

 

Last revision date: <21 Feb 2014>