Protection from UVR by Clothing

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 1979-94, although there is now evidence from several countries that the mortality rate due to malignant melanoma is beginning to stabilise ^{1, 2}. The incidence of malignant melanoma in women in Scotland has also stabilised since 1986 ².

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 ³. The CPF is also analogous to the Sun Protection Factor (SPF) quoted for sunscreens. A British Standard ⁴ describing a suitable method for the measurement of CPFs has recently been published. NRPB played a substantial role in the development of the standard and makes many CPF measurements for companies in the clothing industry.

Determination of CPF

In order to calculate the CPF provided by a fabric, its spectral transmittance must be measured over the wavelength range 290-400 nm. The spectral transmittance is the fraction of incident UVR penetrating the fabric at each wavelength. NRPB 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 five 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 ⁵. They are then summed over the wavelength range 290-400 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.

The fabric is assigned a CPF category based on the lowest measured CPF, as shown in Table 1. As with SPFs, a higher CPF represents a higher level of protection. A CPF of 20 reduces the exposure to erythemally effective UVR by a factor of 20, a CPF of 10 reduces exposure by a factor of 10, and so on. Fabrics with CPFs less than 10 do not provide adequate UVR protection for summer outdoor use. CPFs much higher than 30 are often measured on dense fabrics. At high CPFs, a large change in CPF produces only a small change in the effective transmittance of the fabric. For example, an increase in CPF from 50 to 100 is associated with a decrease in effective transmittance from 2% to 1%. Since this improvement in CPF gives a false impression of a large reduction in transmittance, and because measurement equipment becomes less reliable at low transmittance, the highest CPF category assigned at present by NRPB is 30 .

Table 1

Minimum CPF	CPF category	Protection	Erethymally effective UVR transmittance (%)
> 30	? 30	very high	? 3.3
20-29	? 20	high	3.4-5.0
10-19	? 10	medium	5.1-10.0
< 10	< 10	low	> 10

The CPF of a fabric is changed by effects such as stretch or wetness. Its CPF may also change after repeated wearing and washing. It is important to assess how stretch and wetness affect the CPFs of fabrics, especially those commonly used in leisurewear and swimwear. NRPB can measure CPFs under both stretched and wet conditions.

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. Table 2 shows CPF measurements from three different fabric types, each available in several colours.

Table 2

Fabric	Colour	Minimum CPF	CPF category
woven 100% cotton	cream	7	< 10
	bright pink	73	? 30
	turquoise	100	? 30
knitted 100% cotton	off-white	9	< 10
	white	57	? 30
knitted polyester/lycra	yellow	96	? 30
	orange	136	? 30
	blue	169	? 30
	navy blue	178	? 30

Darker coloured fabrics often absorb more UVR than lighter fabrics, but the protection provided by a fabric cannot be reliably predicted from its colour.

Off-white and cream fabrics often provide lower CPFs, but white fabrics usually offer higher protection. This is because white fabrics usually contain fluorescent whitening agents which absorb UVR and may also be more reflective.

CPFs have been measured on a wide variety of elastic fabrics while the fabric was stretched. In all cases the CPF was reduced. In some cases the fall in CPF was sufficient to place the fabric in a lower CPF category. Both linear stretches (fabric stretched in one direction only) and areal stretches (fabric stretched equally in all directions) have been tested. Table 3 shows CPF measurements under 30% areal stretch on one group of fabrics from Table 2. The exact fall in CPF depends on how the structure of a fabric opens under stretch.

Table 3

Fabric	Colour	Minimum CPF	CPF category
knitted polyester/lycra	yellow	21	? 20
	orange	35	? 30
	blue	30	? 30
	navy blue	55	? 30

Both increases and decreases in CPF were seen when CPFs were measured on wet fabrics. The largest increases and decreases in CPF were seen on fabrics containing a substantial proportion of cotton.

In most cases the changes in CPF were reversed when the fabric dried but some showed a permanent increase in CPF after drying. This was attributed to shrinkage of the fabric. Polyester and nylon fabrics, many of which also contained elastane, have to date shown smaller changes in CPF when wet. The CPF often falls slightly when wet and returns to close to its original value when dry. Table 4 shows the CPFs measured on a selection of wet fabrics, together with the CPFs when dry before and after the fabric was wetted. These results illustrate the variety of changes in CPF which can be seen as a result of wetting a fabric.

Table 4

Fabric	Colour	Dry CPF (before)	Wet CPF	Dry CPF (after)
woven 100% cotton	cream	7	4	7
knitted polyester	navy blue	24	24	25
knitted 100% cotton	white	33	36	52
knitted 100% cotton	pale pink	48	102	64
knitted polyester/lycra	red	200	175	202
knitted nylon/lycra	red	222	184	213

Summary

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.

Acknowledgement

The Department of Health provided funding for this work, under contract.

1 NRPB. Health effects from ultraviolet radiation: Report of an Advisory Group on Non-Ionising Radiation. Doc. NRPB, 6, No. 2, 7-190 (1995).

2 MacKie, R M, et al. Cutaneous malignant melanoma in Scotland: Incidence, survival and mortality, 1979-94. Br. Med. J., 315, 1117-21 (1997).

3 BS EN 13758:2:2003. Textiles - Solar UV protective properties.

4 BS EN 13758:1:2002. Method of test for apparel fabrics.

5 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).

A further article from the Radiological Protection Bulletin concerning Clothing Protection Factors.

Judith Agnew, Kirstie Grainger, Irene Clark and Colin Driscoll
National Radiological Protection Board, Chilton and Glasgow

First published in the Radiological Protection Bulletin, No. 200, April 1998.
Updated April 2004.

View details about our clothing protection factor measurement service.

Last reviewed: 4 September 2008