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1. Health risks from exposure to ultraviolet radiation (UVR)
Exposure to ultraviolet radiation (UVR) constitutes a significant health risk to people, with the sun being the main source of UVR for most. In the short term, erythema (sunburn) and photokeratitis (snow blindness or arc eye) can cause severe discomfort. Skin cancer is the most serious long-term health effect. The incidence of malignant melanoma, the most serious form of skin cancer, doubled in the UK over the period 1979 to 1994, although there is now evidence from several countries that the mortality rate due to malignant melanoma is beginning to stabilise [footnote 1], [footnote 2]. The incidence of malignant melanoma in women in Scotland has also stabilised since 1986.
As a result of these health risks, the reduction of personal UVR exposure is important to many people. Clothing is an obvious source of protection against UVR exposure, but its effectiveness is not fully quantified. In addition to a garment providing good coverage of the skin, its fabric should prevent most of the incident UVR from reaching the skin beneath it.
Both the structure of a fabric (its fibre content, fibre thickness and the knit or weave) and its colour can have a large influence on its UVR protection. It is not possible for the average consumer to make a reliable assessment of the UVR protection of a fabric by visual inspection, so a method has been developed for determining the clothing protection factor (CPF) provided by a fabric which is equivalent to the ultraviolet protection factor (UPF) described in the British Standard [footnote 3].
The CPF is also analogous to the sun protection factor (SPF) quoted for sunscreens. A British Standard [footnote 4] describing a suitable method for the measurement of CPFs is available. National Radiological Protection Board (NRPB), now part of Public Health England (PHE), played a substantial role in the development of the standard and PHE makes CPF measurements for companies in the clothing industry.
2. Determination of CPF
In order to calculate the CPF provided by a fabric, its spectral transmittance must be measured over the wavelength range 290 to 400 nm. The spectral transmittance is the fraction of incident UVR penetrating the fabric at each wavelength. PHE uses a diode array spectrophotometer for making spectral transmittance measurements. This instrument uses a xenon lamp to irradiate the fabric and measures the spectral transmittance at all wavelengths from 250 to 450 nm simultaneously. It provides much faster measurements than conventional systems which measure spectral transmittance at each wavelength in turn. The spectral transmittance is measured at a minimum of 5 positions on a fabric sample.
To calculate the CPF, the spectral transmittance data at each wavelength are multiplied by the solar irradiance at the same wavelength and by a factor representing the effectiveness of that wavelength in causing erythema [footnote 5]. They are then summed over the wavelength range 290 to 400 nm. This gives the erythemally effective solar UVR penetrating the fabric. The corresponding quantity is evaluated for solar UVR with no fabric in place. The CPF is the ratio of the erythemally effective solar UVR with no fabric in place to that through the fabric.
3. Typical results
A very wide range of CPFs has been observed from different fabrics. Very light weight fabrics with an open structure, such as the very light woven cottons used in a sarong, often have CPFs of less than 5. By contrast, a heavier fabric with a closed structure, such as a knitted fabric containing elastane, may have a CPF of 500 or more. Colour can have a large effect on the CPF of a fabric, as different dyes absorb UVR to different extents. An assessment of the protection provided by children’s clothing has been published [footnote 6].
Clothing fabrics have a wide range of CPFs and those with the lowest CPFs (less than 10) provide inadequate protection from solar UVR during summer conditions in the UK. Laboratory measurements of the fabric transmittance are required to determine a CPF; it cannot be adequately predicted by visual inspection of the fabric. Both fabric structure and colour have a large effect on a CPF.
Environmental factors, including stretch and wetness, can change the CPF provided by a fabric. Many leisurewear and swimwear garments are likely to be worn under these conditions. Stretch causes a substantial decrease in the CPF provided by a fabric.
Wetness can cause either increases or decreases in CPF. The largest changes, both increases and decreases, have been seen on cotton fabrics, but synthetic fabrics have also shown changes. The effect of wetness on CPF must be assessed by measurements on the wet fabric, since it cannot be predicted from the dry CPF and fabric type.
Clothing can provide an effective method of UVR protection, in conjunction with other protective measures such as the use of sunscreens and seeking shade when possible.
NRPB. Health effects from ultraviolet radiation: Report of an Advisory Group on Non-Ionising Radiation. Doc. NRPB, 13, No. 1, 1-276 (2002). ↩
MacKie, R M, et al. Cutaneous malignant melanoma in Scotland: Incidence, survival and mortality, 1979-94. Br. Med. J., 315, 1117-21 (1997). ↩
BS EN 13758-2:2003. Textiles. Solar UV protective properties. Classification and marking of apparel. ↩
BS EN 13758-1:2002. Textiles. Solar UV protective properties. Method of test for apparel fabrics. ↩
McKinlay, A F and Diffey, B L. A reference action spectrum for ultraviolet induced erythema in human skin. CIE J. , 6, No. 1, 17-22 (1987). ↩
Khazova, M, O’Hagan, J B and Grainger K J-L. Assessment of sun protection for children’s summer 2005 clothing collection. Radiation Protection Dosimetry (2007), Vol. 123, No. 3, pp. 288–294. ↩