Reply to Professor Macpherson's critique

Abstract

We are grateful to Professor Macpheron for his critique of our paper and particularly for his constructive criticism of our comments on the weighting given in the P4SR index to the effects of mean radiant temperature (M.R.T.). However, before we deal in detail with Professor Macpherson's critique it is necessary to put into perspective the various aspects of our study because, in Professor Macpherson's opinion, one of our comments on the effects of radiation 'represents the major scientific finding described in the paper'. This is certainly not our view. We feel that because of the undue emphasis Professor Macpherson gives in his critique to this aspect of the paper he has lost sight of the main purposes of the study. There were two main aims of the study in the mine at Mount Isa. The first was to determine which of three methods of assessing heat stress, the W.B., the C.E.T., or the P4SR, predicts most accurately the heat stress in the mine. The second aim was to try to set limits, based upon a rational physiological criterion, of heat stress at which the shift should be curtailed and at which work should be stopped. Professor Macpherson criticizes our attempt to achieve these aims by carrying out a study in the mine. We recognize his preference for hot room studies but the purposes for which the indices of heat stress were introduced was to be able to assess heat stress in industry. Academic exercises in hot rooms have their place but, finally, the accuracy of the indices of heat stress must be examined in industry if they are to be of any practical use. When we plotted oral temperatures against P4SRs we found that the plots for high G.T.s fell well below the rest of the data. This result was quite unexpected but we regard it as an incidental finding. It does, however, raise some important issues with regard to the weighting given by P4SR to the effects of radiant heat and we would have been wrong to have ignored these issues. Professor Macpherson advances two criticisms of our interpretation of the anomalous finding with regard to oral temperatures at high G.T.s. In the first he disagrees with our division of the oral temperature/P4SR data into two class-intervals of differences between D.B. and G.T. This is a common statistical practice and we used a similar approach in examining the accuracy with which P4SR takes account of differences between D.B. and W.B. In his second criticism Professor Macpherson expresses surprise that the mathematical analysis of the relative weighting given to various heat stress parameters by P4SR shows that the effect of increasing D.B. decreases P4SR. A moment's reflection will enable the reader to appreciate that the net effect of an increase in D.B., with all other environmental measurements held constant, would be a lowering of the relative humidity, and, in the range of air conditions examined, this would have the effect of reducing the heat stress as expressed by P4SR. Dealing with our comments on the effect of M.R.T. on P4SR, Professor Macpherson complains that we misquoted Ferres et al. (1954) by inserting in brackets the words 'as measured by G.T.' It would have been better if these words had appeared in italics to indicate that they were inserted by us and if anyone has been misled we apologize. We merely wished to draw attention to the fact that M.R.T. was determined from measurements of G.T., using Bedford's equation, and not by any other method such as a radiometer, for example. Professor Macpherson points out an error in our comments on the conclusion of Ferres et al. and we are grateful to him for doing so. The statement at the top of page 264 should be changed to read :