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As heatwaves become more frequent, intense, and longer-lasting due to climate change, the question of breaching thermal limits becomes pressing. A wet-bulb temperature (T_w) of 35 °C has been proposed as a theoretical upper limit on human abilities to biologically thermoregulate. But, recent—empirical—research using human subjects found a significantly lower maximum T_wat which thermoregulation is possible even with minimal metabolic activity. Projecting future exposure to this empirical critical environmental limit has not been done. Here, using this more accurate threshold and the latest coupled climate model results, we quantify exposure to dangerous, potentially lethal heat for future climates at various global warming levels. We find that humanity is more vulnerable to moist heat stress than previously proposed because of these lower thermal limits. Still, limiting warming to under 2 °C nearly eliminates exposure and risk of widespread uncompensable moist heatwaves as a sharp rise in exposure occurs at 3 °C of warming. Parts of the Middle East and the Indus River Valley experience brief exceedances with only 1.5 °C warming. More widespread, but brief, dangerous heat stress occurs in a +2 °C climate, including in eastern China and sub-Saharan Africa, while the US Midwest emerges as a moist heat stress hotspot in a +3 °C climate. In the future, moist heat extremes will lie outside the bounds of past human experience and beyond current heat mitigation strategies for billions of people. While some physiological adaptation from the thresholds described here is possible, additional behavioral, cultural, and technical adaptation will be required to maintain healthy lifestyles. 

BACKGROUND:Salt leaching into freshwater is an emerging global environmental health concern. We tested the associations between drinking water salinity and blood pressure, hypertension, and albuminuria. METHODS:We conducted a 2-year panel study in 2022 and 2023 with 434 observations among 327 Daasanach adults aged >18 years in northern Kenya. Water sources were analyzed for overall salinity and ionic composition (sodium; chloride; calcium, potassium, magnesium). We measured resting blood pressure and classified hypertension stage 1 and stage 2. Urine samples were analyzed for albuminuria (≥30 mg/g albumin-to-creatinine ratio). RESULTS:Drinking water salinity was driven by sodium-chloride (mean=162.6 mg/L, SD=77.1), with low concentrations of calcium, potassium, and magnesium (mean=45 mg/L, SD=13.5). Across 2022 and 2023, 40.1% of adults had at least hypertension stage 1, 13.5% had hypertension stage 2, and 42.2% had albuminuria. Using random effects linear and logistic panel regressions fully adjusted for confounders, each 100 mg/L of drinking water sodium-chloride was associated with 4.5 mm Hg (95% CI, 2.4–6.6) and 3.3 mm Hg (95% CI, 2.2–4.5) increases in systolic and diastolic blood pressure, 3.0× the odds of at least hypertension stage 1 (95% CI, 1.49–5.83), 3.6× the odds of hypertension stage 2 (95% CI, 1.93–6.81), and 2.0× the odds of albuminuria (95% CI, 1.28–3.06). Calcium, potassium, and magnesium were unassociated with any outcomes. Hypertension stage 2 (but not hypertension stage 1) was associated with 2.6× (95% CI, 1.19–5.77) the odds of albuminuria. CONCLUSIONS:Drinking water sodium-chloride was associated with resting blood pressure, hypertension, and albuminuria in a population with few traditional lifestyle risk factors for chronic disease. Measuring specific salts in water helps untangle associations with hypertension.