Which condition can decrease relative humidity indoors during winter months?

Heating the indoor air is the correct choice because it causes the air to become less saturated with moisture. When air is heated, its capacity to hold moisture increases, which can lower relative humidity when no additional moisture is introduced into the environment.

During winter, many homes rely on heating systems to maintain comfortable temperatures. However, as the temperature rises, the relative humidity, which is a measure of the current amount of moisture in the air relative to the maximum amount it can hold at that temperature, tends to drop unless moisture is added to the space.

In contrast, increased air circulation can distribute humidity more evenly rather than decrease it. Opening windows frequently can allow cold, dry air in from outside, potentially reducing humidity levels but also risking heat loss. Using humidifiers, meanwhile, directly adds moisture to the air, which would actually serve to increase relative humidity rather than decrease it.