OBJECTIVE: The aim of this study is to analyse the symptomatic response of elderly people to heat burden and indoor air quality exposure, and to create an index, the basis on which healthcare workers could react and prevent heat-related illnesses when the first symptoms appear. METHODS: The impact of the indoor thermal environment was studied with regards to Humidex and indoor air quality by CO2 concentrations on elderly people's symptomatic response. It was a natural experiment in which two different groups of elderly people (>65 years) were observed: the first group had a diagnosed cardiovascular disease, and the second group did not have the disease. RESULTS: The results show that the expression and aggravation of symptoms are related to an increase of heat burden and low indoor air quality. The symptoms under analysis do not have the same frequency distribution of intensity and, therefore, cannot be interpreted as a single universal symptom index. Instead, two indices must be created separately for both general and specific symptoms. CONCLUSIONS: Healthcare workers should be educated about the interactive influences of the thermal environment and the air quality on health. Unsuitable conditions could be ascertained by the nursing home occupants' symptomatic response.