This study examines the impact of heat waves (HWs) on mortality rates in 14 densely populated urban areas in Brazil, representing approximately 35% of the country's population. The analysis focuses on the period from 2000 to 2018, using the observed-to-expected ratio (O/E) to estimate excess mortality during HWs. The study also assesses the interplay of demographic and socioeconomic factors, including gender, age, race, and educational level, in vulnerability to heat-related illnesses. Key findings include: 1. **Long-term Trends**: The frequency of HWs has increased significantly from the 1970s to the 2010s, with higher tendencies in the northern, northeastern, and central-western regions. The duration of HWs has also increased, particularly in the last two decades. 2. **Excess Mortality**: Between 2000 and 2018, approximately 48,075 excess deaths were attributed to HWs, with the most populous regions (Rio de Janeiro and São Paulo) experiencing the highest absolute numbers. Normalized mortality rates were highest in Rio de Janeiro, Porto Alegre, Belém, Cuiabá, and Recife. 3. **Demographic and Socioeconomic Disparities**: Older adults (65+ years) and females were more affected by heat-related deaths, with older adults accounting for 75-94% of excess deaths. Females had higher O/E ratios than males in all regions. Educational level and race also played significant roles, with lower-educated and black/brown individuals showing higher vulnerability. 4. **Leading Causes of Death**: Circulatory and respiratory diseases, as well as neoplasms, were the most frequent causes of heat-related excess deaths. Skin and subcutaneous tissue diseases, nervous system diseases, and genitourinary system-related deaths also showed significant increases in mortality rates. 5. **Event-based Surveillance**: An event-based surveillance analysis using social media data revealed that information about HWs was only recorded from 2014 onwards, with a notable increase in publications from 2016 onwards. However, none of the high-mortality HWs detected by EIOS matched the events with the highest O/E ratios. The study highlights the need for enhanced primary health care and efforts to reduce socioeconomic, racial, and gender inequalities to mitigate the impact of heat-related deaths.This study examines the impact of heat waves (HWs) on mortality rates in 14 densely populated urban areas in Brazil, representing approximately 35% of the country's population. The analysis focuses on the period from 2000 to 2018, using the observed-to-expected ratio (O/E) to estimate excess mortality during HWs. The study also assesses the interplay of demographic and socioeconomic factors, including gender, age, race, and educational level, in vulnerability to heat-related illnesses. Key findings include: 1. **Long-term Trends**: The frequency of HWs has increased significantly from the 1970s to the 2010s, with higher tendencies in the northern, northeastern, and central-western regions. The duration of HWs has also increased, particularly in the last two decades. 2. **Excess Mortality**: Between 2000 and 2018, approximately 48,075 excess deaths were attributed to HWs, with the most populous regions (Rio de Janeiro and São Paulo) experiencing the highest absolute numbers. Normalized mortality rates were highest in Rio de Janeiro, Porto Alegre, Belém, Cuiabá, and Recife. 3. **Demographic and Socioeconomic Disparities**: Older adults (65+ years) and females were more affected by heat-related deaths, with older adults accounting for 75-94% of excess deaths. Females had higher O/E ratios than males in all regions. Educational level and race also played significant roles, with lower-educated and black/brown individuals showing higher vulnerability. 4. **Leading Causes of Death**: Circulatory and respiratory diseases, as well as neoplasms, were the most frequent causes of heat-related excess deaths. Skin and subcutaneous tissue diseases, nervous system diseases, and genitourinary system-related deaths also showed significant increases in mortality rates. 5. **Event-based Surveillance**: An event-based surveillance analysis using social media data revealed that information about HWs was only recorded from 2014 onwards, with a notable increase in publications from 2016 onwards. However, none of the high-mortality HWs detected by EIOS matched the events with the highest O/E ratios. The study highlights the need for enhanced primary health care and efforts to reduce socioeconomic, racial, and gender inequalities to mitigate the impact of heat-related deaths.