Ultraviolet radiation and UV meters

Xeroderma pigmentosum (XP)

Daylight contains:

  • light that you can see
  • ultraviolet radiation (UVR) that you cannot see

UVR can cause cell damage in the skin and eyes. Most of the time, people without XP can repair this damage. 

People with XP cannot repair the cell damage. They have a high risk of any damage turning into skin cancers. The cell damage can also cause freckling and often severe and long-lasting sunburn. 

You can measure UVR in your environment using a UV meter. This is a handheld, battery-operated device. It measures UVR coming through windows and from light sources, such as light bulbs.

Sources of UVR

The sun produces UVR energy. It is also produced artificially (for example, by light bulbs). 

UVR comes from:

  • any type of daylight (any time that the sun is above the horizon, even if it is a cloudy day)
  • some light bulbs, especially halogen and fluorescent bulbs
  • mercury lamps
  • germicidal lamps (electric lamps producing ultraviolet light that kills germs)
  • tanning beds and sunbeds

UVR can travel through windows, including the windows of cars. 

Levels of UVR

The amount of UVR depends on:

  • the time of day (UVR is higher if the sun is higher in the sky)
  • the time of year (UVR is highest during the summer months)
  • location (the sun’s rays are strongest at the equator)
  • height above sea or ground level (altitude) (UVR increases with altitude, as there is less atmosphere to absorb the damaging rays)
  • clouds (heavy cloud cover usually reduces UVR, but does not completely stop it)
  • environment (UVR rays are reflected off surfaces, such as snow, water, sand, concrete and glass)

Types of UVR

There are 3 types of UV.

  • UVA causes skin ageing. It is now thought to contribute to some skin cancers.
  • UVB causes sunburn and skin cancers. It is thought to be the most harmful type of UVR for people with XP.
  • UVC from the sun is mostly absorbed by the ozone layer. This is a layer high above the earth, which protects us from the harmful effects of the sun. There are very small chances of UVC causing damage because of this protection. However, there are also artificial sources of UVC, which you need to avoid.

Using a UV meter

A UV meter is a handheld device used to measure UVR.

There is no agreed ‘safe level’ of UVR. People repair UVR damage in different ways. This depends on if the damage is caused by a one-off high exposure or repeated low exposure. 

We do not know exactly what level of UVR exposure causes a particular amount of skin damage. 

Getting a UV meter

There are different UV meters available to buy. It is important to know exactly which UVR rays (wavelengths) they measure. You can then be sure that they will give you the right information. 

A meter that measures UVA and UVB wavelengths can help you to know your exposure risk in everyday life. 

We recommend trying the UV meter in different lighting conditions, both indoors and outdoors. This helps you to understand what are high and low readings. 

The UV meter measures the levels of UVR that you cannot see. This can be helpful, especially in an unfamiliar area that does not have UVR protection.

Your XP clinical nurse specialist (CNS) can talk to you about UV meters and which ones are suitable for people with XP. They can recommend meters that our medical physics department have tested.

UV meter readings

A UV meter only indicates the UVR levels in a particular place at a particular time. UV meters are not medical devices, so need to be used with good judgement. 

A zero reading does not mean that a place is completely safe from UVR, but shows a very low UVR reading (less than 1 uW/cm2).

If your meter has a reading of 1 or above, you need to protect all exposed areas of skin from UVR. For example, you can use sunscreen, a face visor (a full covering worn over the face) and protective clothes.

If your meter stops working or you think that it is faulty, please contact your XP CNS. 

Resource number: 3512/VER4
Last reviewed: December 2021
Next review due: December 2024

Contact us

If you have any questions or concerns about XP, please contact the XP clinical nurse specialist (CNS).

Phone: 020 7188 6339 or 020 7188 6351, Monday to Friday, 8am to 4pm

Out of hours, please leave a message on the answer phone with your contact details. Someone will return your call within 3 working days.

UV meters and meter checks

For advice on UV meters and for meter checks, please contact the medical physics department.

Phone: 020 7188 3811

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

Is this health information page useful?