As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.
Where vegetation is sparse, temperature is hot. (Image: NASA) An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. According to the EPA, many U.S. cities have air temperatures up to 10Â°F (5.6Â°C) warmer than the surrounding natural land cover.
There's history over hundreds of years of people dying of heat. Philadelphia in 1776 had a major heat wave that caused deaths. We're also living to older ages, and we're more urban now than we have been in the history of the human species. That intense crowding can combine with the heat island effect in big cities.