Introduction – Every summer, an increase in healthcare utilization due to heat-related causes, such as heatstroke, is observed in mainland France. A better understanding of these impacts is necessary to inform prevention policies. One key question concerns how to characterize heat exposure: should we consider only temperature, or should we also take other meteorological variables into account? Methods – This study compares the influence of four thermal indicators (mean temperature (Tm), wet-bulb globe temperature (WBGT), the heat index (HI), and the universal thermal climate index (UTCI)) on emergency department visits in 33 mainland French departments during the summers (June–September) from 2015 to 2019. Five causes were studied: hyperthermia, dehydration, hyponatremia, isolated fever, and malaise. The analyses are based on quasi-Poisson regressions. Results – For all indicators, heat is associated with an increased risk of seeking emergency care for hyperthermia, dehydration, hyponatremia, isolated fever, and malaise. The relative risks (RR) are of similar magnitude across indicators, except for HI. For example, for hyperthermia, an increase in exposure corresponding to the interquartile range is associated with an RR of 3.97 (95% confidence interval, 95% CI: [3.82–4.13]) with Tm, 3.81 [3.67–3.95] with WBGT, 4.22 [4.01–4.33] with UTCI, and 1.22 [1.21–2.23] with HI. Conclusion – The results confirm that in mainland France, Tm is a good indicator for characterizing heat in terms of its health effects. More complex indicators that include other meteorological variables do not provide a better explanation of these effects. Studies documenting exposure circumstances would be useful for better understanding the value of these exposure indicators in specific contexts, such as sporting events.