First published June 2017; updated May 2019
What is solar ultraviolet radiation?
Ultraviolet (UV) radiation is one of the three major components of sunlight, which include visible light and infrared (heat). It is divided into three categories, classified according to wavelength: UVA, UVB and UVC. The ozone layer blocks all UVC and most (90% or more) of UVB, while UVA passes through the atmosphere relatively unchanged. Therefore, the two types of UV radiation that we need to be concerned about, are UVA and UVB.
Why is UV radiation important?
UV radiation in sunlight has both positive and negative health effects. Although UVB plays a role in vitamin D production, UV radiation can also damage skin cells, with overexposure causing both visible (sunburn) and invisible damage, resulting in premature ageing, eye damage (including cataracts) and increasing skin cancer risk. Furthermore, vitamin D produced in the skin is ‘biologically regulated’ – long periods of sun exposure can actually break it down, therefore reducing benefit.
UVA & UVB
Longer wavelength UVA rays penetrate more deeply through the layers of the skin than UVB. It is associated with skin ageing as well as skin cancer. UVA can pass through window glass and is present all year round, even on cloudy days. Shorter wavelength UVB rays are mainly responsible for sunburn, can’t pass through window glass and are strongly associated with two types of skin cancer – malignant melanoma and basal cell carcinoma.
A worrying trend
Skin cancer is the most common cancer in Ireland, with rates projected to more than double by 2045. Yet in most cases, it is preventable and early detection leads to better outcomes. The vast majority of these cancers are caused by overexposure to UV radiation, mainly from sunlight, although UV radiation from artificial sources (e.g. sunbeds) can also cause skin cancer.
Most people living in Ireland have fair skin which burns easily and tans poorly, so are particularly vulnerable to sun damage and skin cancer.
What influences UV level?
UV radiation levels are influenced by many factors, including:
- Latitude: The closer to the equator, the greater the UV radiation levels.
- Time of day: Solar UV levels change throughout the day, rising during the morning and reaching their peak when the sun is at its highest point in the sky above the horizon (solar noon), before declining during the afternoon and evening. Approximately 60% of UV is received between 10am and 2pm.
- Time of year (Season): During winter, the sun is lower in the sky than during the summer months. The length of the day is also shorter, and so UV is less intense.
- Altitude: UV levels increase by 4% for every 300 metre rise in altitude.
- Reflection: UV is reflected from many surfaces, for example sea surf (white foam) can reflect about 25-30% of UV, while a concrete footpath can reflect up to 12% of UV.
- Cloud cover: While UV levels are highest when skies are cloudless, over 90% of UV can pass through light cloud.
- Ozone: the ozone layer provides a defence against UV penetration. Ozone levels can vary over the year; lower ozone values lead to higher UV levels, while higher values lead to lower UV levels.
Protect yourself from overexposure
UV radiation cannot be seen or felt (infrared radiation causes heat, not UV) so you need to defend yourself against overexposure.
The Global Solar UV index is a scale that was developed by the World Health Organisation which measures the UV radiation level at the surface of the Earth, and gives an indication of the potential for skin damage. The UV index ranges from zero upwards – the higher the UV index, the greater the risk.
When the UV index is 3 (moderate) or above, you need to protect your skin.
Home or away
In Ireland, make sun protection part of your daily routine particularly from April – September, when the UV index is usually 3 or above, even when it is cloudy. But remember, whether at home or abroad, if the UV index is 3 or above, protection is required.
We want everyone in Ireland to learn to Protect & Inspect their skin! Read our short guide, written with hospital-based dermatologists, to checking your skin.