Document Z475mqg9ZVYnYZN9mEvOm69K8

110 ROSS A. MC FARLAND limits of comfort, discomfort, and physiological harm are indicated for several variables. This chart was prepared specifically with reference to commercial vehicle design and operation but may have some applicability in other industries. A. TEMPERATURE AND HUMIDITY In view of the evidence indicating the effect of heat stress on accident rates in various types of work, temperature control for the working area must be con sidered a necessary part of any program in human engineering. For moderate to heavy manual labor, Vernon, Bedford, and Warner have shown that accident rates among mine workers bear an orderly relation to temperature. Near 60 F. acci dents were at a minimum, with two- to threefold increases at temperatures approxi mating 80 F.9< In light manual work involving an element of manual dexterity, Osborne and Vernon conducted studies in British munitions factories, that show that both cold and heat stress increase the frequency of accidents in manual opera tions concerned with shell manufacture.85 Accidents were at a minimum at 67 F. and increased both above and below this temperature. It is to be assumed that the 35 per cent increase noted at 50 F. is an effect primarily on manual dexterity. The increase with higher temperatures may reasonably be attributed to decrease in general alertness and to increased physiological stress of uncomfortable heat; In skilled work involving minimal physical exertion, Mackworth has shown that during long periods of work, errors increased from an average of 12 per hour to more than 90 per hour as effective temperature was increased from 79 to 97 F.96 -In-the-sense-that-errors-in-skilled-acts'may-be-considered-accidentsj-such-data^aTe convincing evidence of the deleterious effect of heat stress on human adjustment. It is thus quite clear that control of the heat loss of workers is important in relation to accident susceptibility. In addition, the provision of a Controlled atmos pheric environment is a factor in employee morale. In a general sense, the imposi tion of any stress such as heat or cold will probably produce poorer performance over a long period, which is likely to be characterised by higher accident incidence. To some degree, such a higher accident frequency may reflect both a conscious and an unwitting protest against a subjectively stressful and uncomfortable en vironment. There are, however, some physiological circumstances of an objective nature that can contribute to an understanding of increased accident susceptibility at elevated-temperatures; At high temperatures,.the diversion of a la|ge;^drtion:6f,the heart's output of' blood to peripheral areas for cooling purposes reduces the reserve in the circulatory supply to the brain areaB responsible for work integration and precision in move- ** H. M. Vembil, T. Bedford, and A. G. Warner,' The relation of^atmospHerfc conditions to the working capacity and accident rate of miners, Indus. Fatigue Research Bd. Rept. No. S9, London, H. M. Stationery Office, 1927. " E. E. Osborne and H. M. Vernon, Two contributions to the study of accident causation,' Indus. Fatigue Research Bd. Rept. No. 19, London, H. M. Stationery Office, 1922. " N. H. Mackworth, Effects of heat on wireless operators, Brit. J. Ind. Med., 3, 143 (1946). HUMAN ENGINEERING AND INDUSTRIAL SAFETY 111 jjiriand discrimination, with the attendant possibilities for personal error. The ggjfed'effects of high temperatures on accident rates can be tentatively attrib- jjthis factor. Moderate cold stress, though uncomfortable, is usually asso||J|with an alert meiyt&l condition. Quite probably the accidents associated with rifely cool conditions are related to chilling of the extremities and a general Him manual dexterity or, where falls are involved, to a decline in the precir||p.ot placement.2 ' i advantages of air conditioning should be seriously considered for all Sal operations carried out for any length of time under conditions of high Matures, either atmospheric or environmental in origin. The requirements of jp or passenger comfort frequently necessitate air-conditioning equipment Igioffices, and various formis of transportation. In industry, however, the must sometimes perform his job under conditions of moderate to intense (ffie added expense of such air-conditioning equipment must be considered, uEsSj-.but only in the light of the attendant increase in the safety, health, jjfand morale of the worker. In many industrial operations, the over-all ' is of this equipment could well justify the initial financial outlay. For a discussion of the problems involved in work under conditions of excessive ||hres see Chapters VIII and XXI. B. NOISE AND VIBRATION quantitative data can be presented to the engineer in regard to limits of noise and vibration, an important contribution could be made to the Spit of efficiency, the lessening of fatigue, and the reduction of the possim^^-^ccidents. Those principles related to noise that are especially important If,!njS|tl'egtandpoint of safety may be summarized as follows. (1) Sudden loud .ri^nd> to speed a worker's movements. This effect is noticeable even when ma|i|,unrelated to the worker or his task (for example, a foreman's call bell), ^^S|en more marked when the sound is related toytshe immediate task (for ip^gsignal to proceed with the next operation). Although this principle can ^tjPtoidvantage in some situations, its possible effect upon operator errors ^^5t^e^sofutinized carefully. (#) Sudden loud sounds may also' tend to disrupt TOinwiTi^'! ------------------ ---------- --ct---^ out systematically. (S) Increased sound levels or added noises may Jj^^M^lr-ror rates at least temporarily; and, conversely, decreased intensity ^^Sj^iyclriapp^earance of sounds may reduce error rates. These effects are likely When there is a prevailing belief among the workmen that ^'gis^.ffpts their work adversely.2 Iking effects of one sound by another have a direct bearing upon the |c;"ommunications among workmen. The principles of masking that, are atxit- '^Smipprtanf are (I) as the intensity level of the disturbing sound is in- K; m m