An air bath is a therapeutic outdoor activity that consists of exposing one's body to fresh open air, either nude or with minimum clothing (e.g. only underwear or shorts), at cooler temperatures so that the body experiences a chilling reaction (goosebumps).
The term is not to be confused with an open air bath (which is a bath in water in the open air) or with a bubble bath (which is also a bath in water). An air bath is the exposure of the body to the outdoor air and weather. Air baths were taken in any season, including winter, and in any kind of weather, including wind, rain and snowfall. The chillier the better the hardening effect on the body was thought to be. They were also sometimes called light air baths because exposing the skin to sunlight was thought to be a key element of the method as well.
During the air bath the patients are encouraged to breathe deep and exercise their body, e.g. by circling their arms and legs, jumping, running, and/or rubbing their body with their hands or a towel. Effects are improved circulation, activation of the metabolism, increased hormone production, and stimulation of the autonomic nervous system. Further effects that were hoped for include improved resistance and strengthening the entire body and mind. After the air bath, which should be no longer than 5 to 10 minutes, the person comes inside to warm up quickly so they won't catch a cold. They either dress in warm clothes, or take a hot shower or bath.
Air baths came up as a method in naturopathic medicine in the late 19th and early 20th century and, like cold water therapies, were believed to be very healthy. They were popular in many European spas, health resorts and rehabilitation centers, but came out of fashion in the 2nd half of the 20th century.
In health resorts for children, groups of patients would be organized to take regular air baths (e.g. every morning) either stark naked or clad only in their underwear, e.g. running laps in the park or garden, or around the building, before they could get back inside to warm up. In autobiographies, some of the patients describe how they felt such treatment harsh, embarrassing and/or humiliating.