Bath water is heated to warm the body, and in Japan it is used to remove cleaning agents from the skin before entering the bathtub. It is assumed that the water does not enter the body during bathing. However, in a shower, water can be unintentionally inhaled as an aerosol. Water could also enter the body through wounds in the skin. In public baths, used water may not be replaced with fresh water for several days, and this could result in spread of infectious diseases. Bacteria can grow in water heaters if they are not cleaned properly, and the addition of inorganic compounds to the bath water from the water source or the skin of bathers can be thought to promote bacterial growth.
Many bacteria can grow in soil, and in a bathroom, bacteria in the bath water will circulate throughout the water heater where they could adhere and grow. Consequently, filtration of water before it enters the water heater is a requirement at large public baths in Japan. However, many home bathrooms in Japan are equipped with small cyclical type boilers that re-heat bath water without filtration, and bacteria could multiply in these systems.