Definitions

• Humidity is a mixture of water vapor and air.Jupiterimages/Photos.com/Getty Images
  
  Humidity refers to the amount of water vapor in the air. When meteorologists measure the amount of water vapor in a specific volume of air, that measurement is called absolute humidity. Absolute humidity is also called specific humidity.
  
  Relative humidity is a measurement of the humidity relative to the temperature. It is expressed as a ratio, or a percentage, that indicates how much water vapor is in a particular volume of air relative to the maximum amount of water vapor that particular volume of air can hold at that particular temperature. That maximum is called the saturation point.
  
  

Highs and Lows

• The low relative humidity of desert climates makes the heat feel more bearable.Jupiterimages/Photos.com/Getty Images
  
  At higher temperatures, the air can hold more water. When the relative humidity is high, it feels hotter outside, because there is more moisture in the air. When the relative humidity is low, it feels less hot at the same temperature because there is less moisture in the air.
  
  In winter, high relative humidity makes cold temperatures feel even colder. That may seem counterintuitive, but it's the same principle. When it's cold, the air cannot hold as much water as it can at higher temperatures -- but when the relative humidity is at or near the saturation point for that temperature, it makes the same cold temperature feel even colder than it would if the relative humidity were lower.
  
  

Low Humidity and Your Health

• Cold, dry air can aggravate symptoms of respiratory conditions like asthma.Tom Le Goff/Digital Vision/Getty Images
  
  The signs of high humidity are well known and include: excessive sweating, feeling hot and sticky, and the danger of heat stroke. However, low humidity can be equally problematic. Low relative humidity (defined as under 30 percent) can cause or aggravate dry and cracked skin, sinus problems and chapped lips.
  
  Low relative humidity in indoor environments, as well as the heat levels we maintain indoors in cold weather, dries out the mucous membranes that defend against colds and flu and other respiratory diseases. Dry nasal passages can also worsen symptoms of asthma.
  
  

The Best Defense

• One of the best ways to defend against the adverse effects of cold, dry air is to use a humidifier. By increasing the moisture in the air, humidifiers can prevent or ease dry, cracked skin and provide relief for the symptoms of colds, flu and chronic respiratory conditions such as asthma.
  
  The beneficial effects of humidifiers are best achieved through proper maintenance. If the humidifier is not kept clean, it can become a breeding ground for disease-causing bacteria and molds that can actually cause illness and make allergies and respiratory conditions worse. It's also important to keep the humidity level in your home at a healthy level -- neither too high nor too low. According to the Mayo Clinic's website, the humidity level should not go below 30 percent or above 50 percent.
  
  Some humidifiers come with a built-in "humidistat" that you can set to your preferred level. You can also purchase a hygrometer: a humidity "thermometer" that measures the humidity level in your home, just as a thermostat would measure the temperature.