Guidelines to monitor workers exposed to heat have been promulgated by the National Institute for Occupational Safety and Health (NIOSH), the American Conference of Governmental Industrial Hygienists (ACGIH) and the Environmental Protection Agency (EP A). In addition the Occupational Safety and Health Administration (OSHA) has developed a technical manual for employers. The adequacy of these guidelines to protect workers of various ages and health status needs further evaluation. We examined the effects of heat on a group of middle-aged construction workers. In addition to monitoring heart rate, temperature and weight as recommended by existing guidelines, we examined neurobehavioral responses, urine osmolarity and pH, blood pressure and symptoms. Baseline characteristics of the 25 participants were that 18 were men, 7 were female; 20 were white, 3 were Hispanic, I was Asian, and none were African-American; all had completed high school with 13 having at least some college education; age ranged from 25-56 with a mean of 40.3 years; 10 were obese (greater than or equal to 30 body mass index (BMI)), and 10 were over-weight (greater than or equal to 25 BMI). There were 7 current smokers. Seven individuals indicated they never drink alcohol. Only 1 worker had ever had to be treated for problems with heat. When working in heat in the past, ten reported sometimes and 7 regularly feeling faint, 10 reported sometimes losing consciousness, 11 reported sometimes and 2 regularly having blurred vision, 11 reported sometimes and I regularly having nausea and vomiting, and 10 reported sometimes and 2 regularly having had a growing feeling of panic. We considered the one individual who had seen a doctor for heat problems and anyone who reported any symptoms on a regular basis to have had history of problems. By this definition, 16 had no history of problems and 9 had. Twenty-three said they tolerated heat better and 2 said they more uncomfortable than most others. None said they easily and quickly get sick from heat. Ten said they knew more than most other people about the effects of heat and 15 about as much as anyone else. None said they were expert or did not know very much about the effects of heat. Eleven participants said they had some chronic disease (5 heart disease, 2 asthma, 2 cancer, 2 high blood pressure, 2 hepatitis, 1 chest operation). Two individuals had multiple conditions. One worker reported not being in good health but reported no chronic conditions. There was no correlation between average skin temperatures and changes in pH or specific gravity between pre and mid shift measurements or pre and post shift measurements. However, there were correlations ofr =.39, (P=.0468) between the average morning wet bulb globe ambient temperature and an increase in the urine specific gravity, and a correlation r =.47 (P=.007) between the average daily wet bulb glove temperature and a decrease in the urine pH. For pre-post shift there was a significant correlation (r=.3 1, P=.0495) between the average afternoon wet bulb globe temperature and a decrease in the urine pH. No correlation was found between morning or daily average wet bulb globe temperature and average morning or daily pulse, systolic or diastolic blood pressure. Similarly, there is no correlation between afternoon and daily average wet bulb ambient temperature and afternoon average pulse, systolic or diastolic pressure. Finally, there was no correlation between the morning, afternoon or daily ambient wet bulb globe temperature and the daily average pulse, systolic or diastolic blood pressure. An inverse correlation was found with average morning and average daily skin temperature and average morning diastolic blood pressure (r = -.47, P = .0 114 and r = - . .55, P =.001, respectively). An inverse correlation was found with average daily skin temperature and average afternoon diastolic blood pressure (r = -.4057, P = .0322). Finally, an inverse correlation was found with average daily skin temperature and average daily diastolic blood pressure (r = -.4057, P = .0322). No significant correlations were found with systolic blood pressure or pulse and morning, afternoon or daily skin temperature. This study was partially successful in demonstrating the ability to collect data on the potential effects of heat among an average working population. The study demonstrated it was technically feasible to collect the data but that full administrative support from the employer is necessary to carry out a study with this level of complexity of data elements. Because of small sample and inadequate number of individuals working on hot days we are unable to demonstrate marked changes in relation to heat exposure. Changes were as expected with a correlation between measures of heat and an increase in urine osmolarity, decrease in urine pH and decrease in diastolic blood pressure. Relatively few people became symptomatic during their work in heat. Further work in assessing heat exposure in average workers under actual field conditions are needed to obtain sufficient sample size to reach meaningful conclusions about the adequacy of existing guidelines for protecting heat exposed workers.