Best Indoor Humidity Range
The ideal humidity range is one that takes into account all of a home's inhabitants, both living and inanimate. Humans, books, and electronics all have their own particular ideal humidity range; fortunately, these ranges overlap. The best humidity range is the area where all three overlap. (The best ways to control humidity are discussed towards the end of this article.)
Before discussing the ideal humidity range, though, it's important to define units. When we talk about humidity, we usually mean relative humidity: the ratio of water vapor currently in the air to the maximum water vapor at that temperature, multiplied by 100. Air at higher temperatures can hold more water vapor, so the air with equal amounts of water vapor at 32o F and 90o F will have drastically different relative humidity. This becomes an issue in the winter, when heating systems warm up air but don't add any water vapor. This makes the relative humidity very low. On the other hand, cooking, bathing, and other living habits all may substantially increase humidity. Air conditioners are typically designed to lower relative humidity to between 30-60%, due to increased evaporative cooling at those ratios.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the ideal humidity range for humans is between 30 to 60 percent relative humidity. The ideal is somewhere around 45-55%. Very high levels of humidity contribute to the growth of mold, funguses, dust mites, and other pests. Mold contributes to a number of diseases and thrives in humid climates, generally above 60 percent humidity. For people suffering from asthma and other respiratory disorders, humidity should not exceed 50 percent, as high humidity can aggravate symptoms. Dry or itchy skin conditions are aggravated by low humidity, which tends to dry out the skin.
Though books aren't alive, they're made out of a formerly living material (wood), and so are subject to degradation. High levels of humidity can allow mildew and destructive pests to damage books, while low levels of humidity can make the paper pages brittle. Brittleness can occur at humidity levels as low as 40 percent, while mildew grows at levels over 50 percent RH. The range recommended by experts is between 30 and 50 percent RH, and temperatures no higher than 70 degrees Fahrenheit.
Image: Flickr - Andrew Ratto - Mildew on Walls
Similarly to books, electronics are sensitive to extremes of humidity. High humidity can alter the conductivity in the devices, leading to damage and malfunction, and possibly corrosion. Condensation becomes a very real problem at high humidity. Very low humidity, on the other hand, can cause the components of a device to become brittle. Electronics are usually designed for operation in normally-experienced humidity ranges, and so will operate correctly in the healthy human range of 30 to 50 percent.