Even a warm day increases the risk of dangerous heart diseases. In hot weather the number of white blood cells (monocytes and killer T cells) increases, heat appears to trigger inflammation and an immune response.
This could make the person more susceptible to infections. The study was based on blood samples from 624 participants in the Louisville Green Heart Project, which studies the influence of green neighborhoods on air quality and the occurrence of heart disease. The average temperature on the days the blood samples were taken was 24.5 degrees.