Critical environmental limits are those combinations of ambient temperature and humidity above which heat balance cannot be maintained for a given metabolic heat production, limiting exposure time and placing individuals at increased risk of heat-related illness. The aim of the present study was to establish those limits in young (18-34 yr) healthy adults during low-intensity activity approximating the metabolic demand of activities of daily living. Twenty-five (12 men/13 women) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress was performed with either (1) constant dry-bulb temperature (Tdb) and increasing ambient water vapor pressure (Pa) (Pcrit trials; 36, 38, or 40 °C), or (2) constant Pa and increasing Tdb (Tcrit trials; 12, 16, or 20 mmHg). Each subject was tested during MinAct and LightAmb in 2-3 experimental conditions in random order, for a total of 4-6 trials per participant. Higher metabolic heat production (p < 0.001) during LightAmb compared to MinAct trials resulted in significantly lower critical environmental limits across all Pcrit and Tcrit conditions (all p < 0.001). These data, presented graphically herein on a psychrometric chart, are the first to define critical environmental limits for young adults during activity resembling those of light household tasks or other activities of daily living, and can be used to develop guidelines, policy decisions, and evidence-based alert communications to minimize the deleterious impacts of extreme heat events.