Introduction
For forty years, Occupational Hygienists have used a variety of brands of cotton gloves as sampling gloves to estimate dermal exposure to the hands. Many brands of sampling gloves have been reported, but the selection criteria are rarely detailed. The sampling gloves are mainly selected for low analytical background, but local availability appears to be a major consideration. The gloves are made for purposes other than sampling, e.g. pall-bearers gloves, photographer's gloves, disposable work gloves, lining gloves. Some gloves are bleached to improve appearance, but cheaper ones are unbleached. Most contain some residues of fabric treatment such as oil to assist weaving or knitting.

When used as sampling gloves, they can used in three ways:
a) outside the protective glove to measure potential dermal exposure; the amount retained on the sampling glove is assumed to represent the challenge to the protective glove.
b) directly on the hand when no protective glove is worn; the amount retained on the sampling glove is assumed to represent the amount falling onto the skin.
c) inside the protective glove (directly on the hand) to measure actual dermal exposure; the amount retained on the sampling glove is assumed to represent the amount falling onto the skin due to the limitations of the protective glove. It is often compared with a) to calculate the protection offered by a glove.

Contamination can collect on the sampling glove through several mechanisms:
d) direct contact with a contaminated surface; the removal efficiency from the surface may be very different to the skin or a protective glove.
e) direct deposition from the air; spray or dust settles onto the exposed sampler but the fibres may retain more than relatively smooth skin.
f) immersion into a contaminant; the large quantities retained in the fibres may bear little relation to that retained on skin.

Guidance, provided for example by WHO and OECD, specifies that samplers be changed for fresh ones as soon as they are saturated, although this bears no relation to the sampling properties of the skin, and it is almost impossible to determine whether a sampler is saturated or not. Certain parts of the glove will always saturate first, usually the fingertips.

There is little information to relate the amounts found on the sampling gloves to the amounts that would have been retained on the skin of the hands in their absence. Limited studies have been conducted to compare hands and sampling gloves, but the issue has been raised as an aside to field survey work. It is generally accepted that oversampling must occur, but for occupational hygiene surveys and subsequent modelling, sampling efficiency is assumed to be the same as the skin for lack of any comparison data. It is not generally appreciated that the results might have been quite different had a different brand of sampling glove been selected. It is an issue that is often ignored.

Most of these cotton gloves are of low manufacturing quality. For the purpose of dermal sampling, there are no quoted standards for quality control that they are required to meet, either for the raw materials or the construction. Lack of quality control of materials could result in batch-to-batch variability, and could affect the consistency of the sampling rate. Pre-treatment of the gloves, such as washing to remove interfering background levels or to increase absorbency, could also affect their sampling properties and possibly increase variation.

Aims of this work
· To obtain a selection of types of sampling gloves used by occupational hygienists throughout the world.
· To measure their sampling efficiency relative to the skin.
· To enable occupational hygienists to relate their past results obtained using gloves, to the sampling properties of the skin.
· To develop a specification for a material that could be widely used as a sampling glove, that would replicate the transfer and retention properties of the skin.

Methods
1. Identification of gloves
Ten brands of glove were obtained that represented reported use worldwide by searching literature of occupational hygiene studies and following through to a supplier. Batches of the gloves were washed in non-biological washing powder in an automatic washing machine because several of the studies reported that this was done to reduce background levels. Tests were carried out on washed and unwashed gloves.

2. Liquid retention Saturation tests were performed by dipping gloves (unworn) and ungloved hands into a beaker of water. The hands were removed after a few seconds, but the gloves were tamped for one minute with a glass rod to allow the fibres to take up the liquid before removal and one minute's drain back into the beaker. The weight loss of water from the beaker indicated the amount retained on the hand or glove.

Lower challenge levels were achieved with a spray mist inside a chamber. Volunteers wore one glove and performed a series of hand movements in the chamber through ports. Retention of spray was measured by rinsing bare hands and gloves separately in pure water, followed by assay for a soluble strontium salt tracer in the spray. Recovery efficiencies from spiked hands was 90-100%, and from gloves was 100% after pre-treatment and acid leach.

3. Dust retention
Four of the ten brands of gloves were tested against chalk dust (calcium carbonate) which according to manufacturer's data was 99.8% pure, and of 28µm median diameter (by seiving). Saturation tests were performed by dipping a gloved or ungloved hand into a large beaker of dust and performing a short series of hand movements to coat the hand or gloved hand thoroughly. The hand or gloved hand was removed, and a further short series of hand movements performed to dislodge loosely-bound dust. The dust was recovered from the hand by bag-washing in water. The glove was carefully removed and bagged over a paper surface, and all the dislodged dust recovered from the paper. The calcium carbonate was dissolved with sufficient nitric acid to clarify the solution, and analysed by assay for calcium. Second bag-washes of the hand showed that no further calcium was removed, so 100% recovery efficiency was assumed from the hands. Spiked amounts of dust on the gloves showed that 100% recovery was achieved. Lower challenge levels have not been started yet, but will be achieved with a dust generator inside the chamber as above.

Results
1. Glove materials
The glove materials were usually reported as 100% cotton, but mixtures were frequently encountered. Cuffs of different materials to the gloves were also found. The materials varied in their size, thickness and weight. Total glove weights were in the range 5-29 g, equivalent to area weights of 0.01-0.07 g.cm-2 of material. Differences were also observed in the weave, texture, absorbency and elasticity.

2. Liquid retention
Retention after saturation ranged from 6.4-62 g per glove, a factor of 3-39 times greater than the hand retention of 1.6-2.3 g per hand. Retention of spray mist was surprisingly constant for the different types of gloves, being approximately twice that of the hands.

3. Dust retention
Retention after saturation ranged from 12-22 g per glove, a factor of 12-40 times greater than the hand retention of 0.5-1.0 g per hand. Lower challenge levels are planned but have not yet been carried out at the time of writing.

Conclusions
Cotton gloves were shown to oversample. At lower challenge levels, sampling efficiency is brand-independent. The higher retention of the glove may be caused by a higher evaporation rate from the fabric than the bare hand. Future tests with dry dusts at low challenge levels can test this theory later. However at saturation, sampling efficiency rises many-fold to a brand-dependent level. The response of the sampling glove depends on the mechanism by which the glove became contaminated - either by deposition directly from the air, or by total saturation from immersion or from partial saturation from contact with contaminated objects.

In field use, there will be parts of the glove that are saturated, for example the fingertips, and parts that are not saturated such as the back of the hand. Past field studies have consisted of an unknown mixture of these brand-dependent and brand-independent sampling efficiencies, and the results have unwittingly depended on the type of sampling glove selected.

It will not be possible to re-assess previous studies in the light of this knowledge unless the deposition mechanism was well-defined. Comparison and collation of different studies that have used different sampling materials must be carried out with a great deal of circumspection.

It is possible to define a specification for a new glove material that mimics the sampling properties of the skin: 410 cm² of it should retain 1.8-2.3 g of water and 0.5-1.0g of dust when saturated, but the material should sample at the same rate as the hand in a controlled deposition study.

General Discussion
This paper deals with gloves, but our lack of knowledge is not confined just to gloves. Looking at the wider issue, surrogate sampling materials used in patch measurements and whole-body measurements are selected mainly to increase sampling efficiency.

Some occupational hygienists take the line that "more is better", preferring to oversample rather than risk undersampling. Dusts are sampled using gauze to trap more particles than would adhere to the skin. Vapours and volatile chemicals are sampled with cotton-carbon cloths that absorb vapours from the air and reduce evaporative losses from splashes, whereas the skin may absorb from the air quite differently and retain only a fraction of the splash.

In a similar fashion, transfer of residues from surfaces to skin are measured using surrogate techniques involving cotton pads or denim cloth. Even those samplers that claim to dislodge similar quantities as the skin have had very limited testing against a limited subset of surfaces.

Before undertaking dermal exposure measurements, researchers should always consider carefully what their measurements are trying to represent.