<P> The human body dissipates heat through perspiration and its evaporation . Heat convection, to the surrounding air, and thermal radiation are the primary modes of heat transport from the body . Under conditions of high humidity, the rate of evaporation of sweat from the skin decreases . Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot dissipate heat by conduction to the air, and a condition called hyperthermia results . With so much blood going to the external surface of the body, less goes to the active muscles, the brain, and other internal organs . Physical strength declines, and fatigue occurs sooner than it would otherwise . Alertness and mental capacity also may be affected, resulting in heat stroke or hyperthermia . </P> <P> Humans are sensitive to humid air because the human body uses evaporative cooling as the primary mechanism to regulate temperature . Under humid conditions, the rate at which perspiration evaporates on the skin is lower than it would be under arid conditions . Because humans perceive the rate of heat transfer from the body rather than temperature itself, we feel warmer when the relative humidity is high than when it is low . </P> <P> Some people experience difficulty breathing in humid environments . Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety . Sufferers will often hyperventilate in response, causing sensations of numbness, faintness, and loss of concentration, among others . </P> <P> Air conditioning reduces discomfort in the summer not only by reducing temperature, but also by reducing humidity . In winter, heating cold outdoor air can decrease relative humidity levels indoor to below 30%, leading to discomfort such as dry skin, cracked lips and excessive thirst . </P>

What is the maximum percentage of moisture by volume in the atmosphere