Robin Howie

Robin Howie Associates, Edinburgh

Clean room clothing is intended to prevent substances released off the wearer's body from contaminating the environment. It is also important that the clothing does not itself release particles or fibres into the environment. Cleanroom personnel can contribute about 25% of airborne particulate contamination (Lieberman, 1992). In the semiconductor and pharmaceutical industries such contamination can degrade product performance, in health care it can result in cross-infection between patients and medical staff.

Skin, hair and other substances are released from the body and clothing with each person releasing about 25,000 particles per minute (p/min). Emission rates in normal clothing range from about 100,000 p/min when sitting or standing motionless, 500,000 p/min during hand, forearm and head movement and up to 10,000,000 p/min during normal walking. The particles released generally range between 5 and 300 um, (Lieberman, 1992). This author has observed skin scales which were indistinguishable from amosite asbestos fibres, (Howie et al, 1996). Fine aerosols can also be released from the breathing tract, a cough producing about 600,000 droplets > 0.5 um in diameter. Such droplets can evaporate to leave particles of body salts, medications and bacteria. Social activities can affect particle emission rates, e.g. Hoenig and Daniel (1984) observed that significantly more particulates were generated by smokers. Each application of cosmetics can involve up to 1,000 million particles. As these particles may not be completely removed during washing and may contain medication and traces of metallic contamination, cosmetics can be a significant source of particulate contamination, (Lieberman, 1992). Particle release rates can increase following sun bathing due to drying of the skin. In addition, many particles emitted from the body can be contaminated by bacteria. A number of authors have identified the clean room worker as a major source of contamination, e.g. (Gunawardena et al, 1984, Hoenig, 1983, Portner et al, 1965, Roderer, 1983, Austin, 1966). The last author commented that the "clean room employee is determined to be potentially a 2 billion particle emitter".

Clothing protective performance can be limited by penetration through fabrics, seams and fasteners and by leakage between the garment and the body at seals at neck or wrist etc. Face-mask performance can similarly be limited by penetration of the filter and leakage between mask and face. Penetration can be regarded as passage through holes in the protective equipment. Such holes can occur in fabrics, filters, seams or fasteners. Leakage can occur at any point at which there is a gap between the equipment and the wearer's body or face.

Clothing penetration and leakage result from pressure differences between the inside and outside which are generated by body movement, e.g. when raising and lowering the arm voids are created between the body and the clothing as the arm is raised and destroyed as the arm is lowered. Air from the environment is drawn inwards as the voids are created and expelled as the voids are destroyed. Airflow into the garment can contaminate the body and airflow out of the garment can contaminate the environment. The pressure differentials generated, and thus the transfer of contaminants, depend on the air permeability of the garment and the goodness of fit, e.g. Whyte and Bailey (1985) observed peak pressure differentials of 5 Pa for an air permeable polyester and 30 Pa for air impermeable "Goretex". Pressure differentials across face masks can exceed 100 Pa due to inhalation and exhalation.

Penetration and leakage of garments and face masks therefore define the ability of the equipment to protect either the body or the environment.

Notwithstanding the importance of clean room clothing penetration and leakage, no performance standards for clean room protective equipment have yet been prepared by the European Standards Committees although numerous informal tests have been used for many years. It is therefore necessary to infer the performance of clean room equipment from workplace and laboratory tests for clean room and protective equipment on the assumption that equipment which prevents the ingress of contaminant from the environment into the protective integument will likewise prevent egress of contaminant from within the integument. This assumption may be valid for clothing but is not valid for some types of face masks.

The type of garment selected should reflect the clean room and product specifications, e.g. for Class 10,000 clean rooms, simple smocks, head covers and booties may be adequate whereas for Class 10 clean rooms careful gowning procedures allied with a zipped coverall, boots, gloves and complete respiratory enclosure will be required.

For effective protection of both the environment and the body, coverall fabric penetration, garment leakage and the thermal characteristics of the garment are all critical.

Conventional clothing fabrics such as cotton, nylon, terylene or spunbond have numerous small holes through the fabric. Objective tests of such fabrics using respirable asbestos fibres or small particles indicate that over 50% of such particles penetrated through the fabrics, this author, unpublished. Whyte and Bailey (1989) tested fabrics from a number of clean room garments against particles > 0.5 um in diameter and reported penetrations ranging between 25% to more than 90%. Fabrics giving such test results are essentially "transparent" to fine aerosols such as bacteria, skin scales or evaporated sweat and therefore unsuitable unless only marginal performance is required.

However, modern performance fabrics such as "Goretex", "Tyvek" or Kimberley Clark "CPF" can afford high performance.

The Table below permits comparison of conventional and high-performance fabrics.

As can be seen, both conventional fabrics permit at least an order of magnitude greater penetration than the high performance fabric, penetration decreases with increasing particle size and the fall in penetration with particle size is relatively smaller for the poorest fabric, (Howie, unpublished).

Unsealed stitched seams and fasteners such as Velcro or conventional zips offer little resistance to either airflow or aerosol penetration. In many garments, the air permeability of seams and fasteners is much greater than that of the fabrics. Consequently, although the area of seam and fastener is typically less than 1% of the total area of a garment, the total flow of contaminated air through seams and fasteners can be much greater than that through the fabric, particularly if the fabric is relatively impermeable to air. For example, Weeks & McLeod (1982) reported the following asbestos fibre penetration results for tests on unstitched and stitched samples of two fabrics:

From the above it can be seen that penetration through seams can be substantially greater than that through the fabric. Similar effects are observed for fasteners such as unprotected Velcro or zips.

In addition, it is necessary to consider leakage between the clothing and the body.

Many garments have gaps between the clothing and the body, most commonly at the neck. Contamination can readily pass through such gaps. Penetration through seal gaps can permit unhindered passage of large particles which would otherwise be unable to pass through the fabrics of construction.

The importance of leakage on total performance has been recognised for many years, e.g. Home Office (1937) warned that the inside of protective clothing impervious to the contaminant could rapidly be contaminated ''owing to the suction effect produced by movement''.

More recently, Fenske (1988), measured pesticide deposition inside protective clothing and observed that a garment effectively impervious to the airborne contaminants only reduced contamination of the wearer's torso by about 50% and, that by drawing contaminated air into the garment, could actually increase contamination of the torso as compared when the "protective" garment was not worn. A similar potential increase in exposure when protective clothing was worn by a panel of subjects was also reported by van Rooij et al (1993) who observed that although protective clothing reduced urinary excretion of a pesticide by an average of 35%, excretion was increased for three wearers. Ojanen et al (1992) tested total performance of five different protective garments during herbicide spraying and observed that although the garments themselves were impervious to the herbicide, the garments permitted penetrations of 11 to 33%.

Such studies clearly indicate that contamination of the body can occur even when the clothing worn is effectively impervious to the contaminant. That is, leakage must be occurring between the clothing and the wearer's body. Such leakage therefore permits airborne contaminants to bypass the garment barrier. A garment constructed from an efficient fabric with well sealed seams and fasteners may therefore fail to provide effective protection for either the body or the environment unless it is well sealed to the wearer's body.

Studies of clean room garments indicate similar conclusions. For example, Portner et al (1965) observed that "when the clean room was occupied by personnel, microbial contamination increased greatly even when personnel wore surgical masks in addition to clean room clothing". Weber and Wieckowski (1982) observed that coveralls, hoods and boots still produced half the airborne contamination produced by smocks and boots. Huff et al (1994) observed that cover gown and head covers reduced the contamination level by only 19%.

The importance of very small gaps at seams and fasteners and at garment seals should not be surprising: although the iceberg pierced less than one percent of the underwater area of the Titanic's hull, the ship still sank.

As noted above, a garment which permits inward leakage will equally permit outward leakage. It is therefore likely that clean room garments using the same fabrics and construction techniques as protective clothing will be equally poor at keeping contamination inside the clothing as it was at keeping environmental contamination outside the clothing.

From the above it will be appreciated that it is not adequate to specify clean room clothing in terms of fabric performance only, it is necessary to specify fabric, seam and fastener penetration and suit-seal leakage.

While it might appear that protective performance could be increased by providing better seals, sealing can cause thermal discomfort or heat strain by preventing or reducing air exchange.

The human body is an inefficient machine which converts energy derived from food into work with typical conversion efficiencies ranging from about 25% for a fit person carrying out exercise with the major muscle groups down to almost zero when carrying out static work such as holding weights above the head. All energy derived from food which is not converted into useful work must be lost as heat. The higher the work load, the greater the amount of heat to be lost. For sedentary tasks such as light hand and forearm work, the body has to lose about 100 Watts of heat, for heavy manual work, up to 500 Watts must be lost. Heat can be lost by convection, radiation, conduction, sweat evaporation and by heating and humidifying inspired air. It should be appreciated that only a few joules per gram is lost by sweat dripping off the body whereas the evaporation of 1 gram of sweat in contact with the body can lose about 2,400 joules. Heat is more easily lost in light, open clothing than in well sealed clothing and more easily lost in cold environments than in hot environments. Well sealed garments exhibiting poor air and water vapour permeation can substantially reduce sweat evaporation as sweat evaporation is much reduced in the high humidities present within the garments. Such well sealed garments can cause significant heat storage for moderate or harder work in other than cold environments. Garments which cause thermal discomfort or heat strain may be misworn in an attempt to reduce discomfort, i.e. the fastener may be opened to permit easier flow of air. Any such miswear substantially reduces the protection provided by the clothing.

If sufficient heat cannot be lost, heat is stored in the body and the body's core temperature rises. Moderate heat storage can cause thermal discomfort, which, although not directly health threatening, can cause loss of attention which could result in reduced quality of product. Severe heat storage can progressively result in cramps, coma and death.

Clean room workers often have to wear coveralls, hoods, gloves and face masks which substantially reduce the area of skin off which heat can be lost by convection, radiation or sweat evaporation. In 15^oC, environments, wearing gloves and headcovers reduces the body's ability to lose heat by about 50 Watts. In addition, face masks absorb heat and moisture from the exhaled air which is transferred to the inspired air, thus reducing heat loss from the respiratory tract. Emerson et al (1967) tested six different surgical masks and observed increases in inhaled air temperature and absolute humidity of between 2.8 and 4.9 C^o and 7.9 and 12.6 mg water vapour per litre respectively. That is, mean overall respiratory heat loss was reduced by about 30 Watts per litre of air breathed per second. Face skin temperature was increased by a mean of 2.9 C^o and discomfort levels were increased.

Well fitted, high performance garments, particularly when worn in conjunction with face masks, can therefore so reduce heat loss from the body that heat strain should be much more common than is presently reported.

There are only two possible explanations for the lack of reported heat strain when wearing nominally high performance garments worn during moderate work in cool conditions: heat strain is occurring but is not being recognised as such or, the present garments do not seriously affect the body's ability to lose heat. If the latter is the case, the corollary is quite simply either that the garments are constructed from highly permeable fabrics which do not provide effective protection and/or do not seal to the body and thus permit air flow by leakage. Such penetration and leakage permits heat loss and contaminant transfer between the wearer and the environment. That is, the lack of heat strain may indicate that the garments may not effectively reduce emissions from the wearers' bodies and thus potentially permit contamination of the environment.

Special fabrics designed to minimise particle shedding are widely used in clean rooms. Test methods for assessing particle shedding characteristics are described in ASTM (1973) and Lamb et al (1990). Garments for clean room use should be designed so that stitching and seaming are arranged to ensure that all cut fabric edges are enclosed.