What is relative humidity and how does it affect how I feel outside?
Humidity is somethi­ng we hear about daily in weather reports. Humidity is to blame for that muggy, steam-room feeling you experience on certain summer days.

Humidity can be measured in several ways, but relative humidity is the most common. In order to understand relative humidity, it is helpful to first understand absolute humidity.

Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain.

­Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creatin­g the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.

­ Humans are very sensitive to humidity, as the skin relies on the air to get rid of moisture. The process of sweating is your body's attempt to keep cool and maintain its current temperature. If the air is at 100-percent relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling ­us off. For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius) and the relative humidity is zero percent, the air temperature feels like 69 degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degrees Fahrenheit (24 C) and the relative humidity is 100 percent, we feel like it's 80 degrees (27 C) out.

People tend to feel most comfortable at a relative humidity of about 45 percent. Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level.

One thing that makes winter uncomfortable for humans, even inside a nice warm building, is low humidity. People need a certain level of humidity to be comfortable. In the winter, indoor humidity can be extremely low and the lack of humidity can dry out your skin and mucous membranes. Low humidity also makes the air feel colder than it actually is. Dry air can also dry out the wood in the walls and floors of our houses. As the drying wood shrinks, it can cause creaks in floors and cracks in drywall and plaster.

Types of Humidifiers

There are many different ways to raise the humidity in your home. For example, you can put a pan of water on the stove or on the radiator, or you hang wet towels near a heater duct. But most people use a mechanical humidifier to do the job. Here are the four most popular technologies:

* Steam

KAZ V150 steam vaporizer

Often referred to as a "vaporizer," a steam humidifier boils water and releases the warm steam into the room. This is the simplest, and therefore the least expensive, technology for adding moisture to the air. You can find inexpensive impeller models for less than $10 at discount stores. Another advantage of this technology is that you can use a medicated inhalant with the unit to help reduce coughs.

* Impeller

KAZ V400 impeller humidifier

In this humidifier, a rotating disc flings water at a comb-like diffuser. The diffuser breaks the water into fine droplets that float into the air. You normally see these droplets as a cool fog exiting the humidifier.

* Ultrasonic

KAZ 5520 ultrasonic humidifier

An ultrasonic humidifier uses a metal diaphragm vibrating at an ultrasonic frequency, much like the element in a high-frequency speaker, to create water droplets. An ultrasonic humidifier is usually silent, and also produces a cool fog.

* Wick/Evaporative System

KAZ V3500 evaporative humidifier

The wick system uses a paper, cloth or foam wick or sheet to draw water out of the reservoir. A fan blowing over the wick lets the air absorb moisture. The higher the relative humidity, the harder it is to evaporate water from the filter, which is why this type of humidifier is self-regulating -- as humidity increases, the humidifier's water-vapor output naturally decreases.

Here are some things to keep in mind as you are weighing the advantages and disadvantages of the different technologies:

* Steam vaporizers can be dangerous around children because they can cause burns. They also have the highest energy costs. However, there are no bacterial or mineral concerns with this technology.

* Impeller and ultrasonic designs have low energy costs but raise two concerns. First, if the water gets stagnant, these designs will spray the stagnant water, and any bacteria it contains, into your home. This is why it is important to clean the tank regularly and refill it with clean water when you haven't been running it. Many high-end ultrasonic units therefore have antibacterial features built in. For example, some units use ultraviolet light to kill bacteria.

The second concern is minerals in the water. Impeller and ultrasonic designs send these minerals into the air. If the water in your area contains a lot of minerals, you will notice them as dust. The EPA does not issue health warnings about minerals in the air, but does recommend using low-mineral water (such as distilled water) in your humidifier. Many ultrasonic models feature a demineralization cartridge that filters minerals out of the water to prevent the dust.

* Some humidifiers monitor the relative humidity of the air and will turn on and off as appropriate to maintain a preset level.

* Humidifiers can be installed as small portable room units, or they can be integrated into your furnace for full-house humidity control.

http://www.lenntech.com/calculators/humidity/relative-humidity.htm