The medical work load seems to increase both with heat and humidity, and with cold and rainy conditions. Heat tolerance during exercise is variable and heat intolerance may contribute to collapse and increase medical encounters. Exposure to cold, wet conditions results in increasing incidence of hypothermia in exhausted marathon runners. Finish-line encounters and course dropouts increase as conditions cool and warm away from the most advantageous conditions in the 4.4-15 degrees C (40-59 degrees F) wet bulb globe temperature (WBGT) range. The risk of requiring medical attention and not finishing rises considerably when the WBGT is >15.5 degrees C (60 degrees F). Comparing the correlation coefficients of the Boston Marathon and Twin Cities Marathon data suggests that the risks of medical problems and not finishing are associated with the warmest temperature of the race and not the start temperature. The community consequences of races conducted in hot and humid conditions can be significant, particularly when the WBGT is >15.5 degrees C. The emergency medical systems can be overwhelmed with a surge of patients, some very ill, and the emergency call response times drop to unacceptable levels blocking access for the citizens of the community. With respect to marathon encounters, heat stress increases both the finish-line medical encounter rate and the on course drop-out rate, and seems to increase the incidence of hyponatraemia and heat stroke. Cold conditions increase the drop-out rate along the course and, if associated with wet conditions, also increase the encounter rate.