Development of a draft British Standard: the assessment of heat strain for workers wearing protective equipment

In many industrial situations workers are required to wear personal protective equipment (PPE) in order to protect them from hazards. Although offering protection, it is known that the wearing of some forms of PPE in some conditions may increase the risk of an individual experiencing heat strain. This is because whilst PPE provides a barrier between the worker’s body and the hazard, it will also usually increase the insulation of the body and/or reduce the opportunity for evaporation of sweat from the body. In situations of heat stress, evaporation of sweat is usually the main route of heat loss from the body.Under the Management of Health and Safety at Work Regulations 1992 it is the employer’s responsibility to assess the risk to their workers’ health and safety of the work activities. This will include assessing the risk of heat strain for workers wearing PPE. Heat strain can be measured directly (physiological measurements) but in many situations this is not practical, and therefore different methods for estimating potential heat stress to which the workers may be exposed have been developed. Two such methods are enshrined in British/International Standards concerning ergonomics of the thermal environment. These are:BS EN 27243: 1994, Hot environments – Estimation of the heat stress on working man, based on the WBGT-index (wet bulb globe temperature).BS EN 12515:1997, Hot environments – Analytical determination and interpretation of thermal stress using calculation of required sweat rate.Both these Standards allow an estimation of acceptable thermal environments or exposure times based on the dry bulb temperature, the wet bulb temperature, air velocity and metabolic heat production rate. BS EN 12515: 1997 is more sophisticated (computing likely heat strain using a BASIC program) and, as well as these factors, also takes account of the thermal insulation of any clothing and the proportion of the body uncovered. However, one assumption made by both these Standards is that the clothing worn by the individual is water vapour permeable and BS EN 27243: 1994 also assumes that the clothing is relatively light. Because most forms of PPE either have a higher insulative value than that assumed or are water vapour impermeable, the Standards cannot be accurately applied to workers wearing PPE. There was therefore seen to be a need to develop a British Standard which would allow interpretation of these existing Standards for workers wearing PPE.In 1996 the BSI Standards subcommittee PH/9/1 (Ergonomics – Thermal Environments) commissioned a research programme concerning heat strain for workers wearing PPE. The main purpose of the research was to provide a draft British Standard on heat strain of workers, taking account of PPE worn. The research programme was conducted in close collaboration with a steering group appointed by the BSI Standards subcommittee PH/9/1.This report outlines the research which was undertaken in the development of the draft British Standard entitled ‘The assessment of heat strain for workers wearing personal protective equipment’. It was issued as a draft for public comment on 15th May 1998.The research which was conducted to develop the Standard comprised of a literature review and discussions with experts, a questionnaire survey to potential users of the Standards, and consideration of physiological data collected experimentally and in work situations. The literature review concentrated particularly on filling the gaps in the existing thermal Standards for workers wearing PPE. The use of correction factors for the WBGT index were considered and, based on scientific evidence, included in the draft Standard. Alterations to the Required Sweat Rate calculations (BS EN 12515) were also supported from the findings of the literature review. Additional supporting information was also identified in the literature, such as metabolic heat production rates and thermal insulation values for different forms of PPE, the proportion of the body covered by PPE, the impact of the fit of clothing on heat stress, and the effect of an air supply (e.g. Breathing Apparatus).Two questionnaire surveys were undertaken to determine the needs of potential users of the draft Standard; the two groups of potential users of the draft Standard were identified as manufacturers and health and safety specialists. Questionnaires were sent to 22 manufacturers and 48 health and safety specialists and follow up telephone discussions were undertaken where appropriate. Response rates for each questionnaire were approximately 50%. The results showed that most manufacturers of PPE which could increase the risk of heat stress were aware of this and some provided information concerning heat stress with the items they manufactured, although this was in general limited to a warning. Health and safety specialists reported the conditions in which the risk of heat stress could arise in their organisation, and described what methods they had used to reduce the risk of heat stress. This was primarily introducing work: rest schedules and allowing the workers to take regular breaks, although a small number had attempted to reduce the ambient temperature by shielding radiant heat sources or introducing air conditioning. Respondents were asked which (if any) heat stress Standards they used. The results indicated that BS EN 27243 (WBGT index) and the ACGIH guidance were widely used, although BS EN 12515 and physiological monitoring were not used. However, more recent reported discussions with users indicates that BS EN 12515 may be more widely used than this survey indicated.Limited physiological data obtained mainly from field work was examined to determine the validity of the recommendations made in the draft Standard. Although little data was available, it did support the recommendations made in the draft Standard. However, examination of data in this way also highlighted the difficulties experienced in collecting substantial, good quality physiological data in the field.The information collected by these methods was used to develop the draft British Standard. The draft Standard provides information and data on: The general effect of PPE on heat balance of the body (the ability of the body to maintain thecore temperature within an acceptable range) The effect of specific forms of PPE on metabolic heat production rate The thermal insulation and evaporative resistance of types of PPE The effect of the closure of the garments to the body The effect of the PPE on the proportion of the body covered The effect of an air supply (BA) to the wearerGuidance is given on conducting an analysis of the work situation, taking account of the impact of PPE. Detailed methods of interpreting both BS EN 27243 and BS EN 12515 for workers wearing PPE are given, taking account of the factors listed above. Three worked examples using BS EN 27243 and BS EN 12515 are given in Annex B of the draft Standard.The ways in which the equations in BS EN 12515:1997 (which are the basis of the BASIC program used in the calculation) should be changed to take account of the PPE are also given (Annex D).The draft Standard was circulated and discussed at a UK Clothing Science Meeting held on 1st June 1998. Public comments concerning the draft Standard were invited, and will be received by BS1 by 31 st July 1998. “”