Evaporative coolers, also known as swamp coolers, cool the air in an environment by allowing hot air to pass through wetted filters, which absorb heat. Fans recirculate cooled, humidified air. Evaporative coolers are generally more energy efficient than traditional air conditioning units, and are best used in hot, dry climates. These coolers are easy to maintain, and as an added benefit, may help to trap dust. Indoor evaporative coolers should be used in well-ventilated areas.