It is important to maintain a healthy level of humidity, which can be achieved through mechanical ventilation equipment.
Dust mites and mould love moisture. Keeping humidity around 30%-50% helps keep them and other allergens under control. A dehumidifier (and air conditioner during summer months) helps reduce moisture in indoor air and effectively controls allergens. A humidifier can be helpful during winter or in hot climates, raising the level of humidity back within the range, when dropped too low.
An air conditioner with a high-quality filter also reduces indoor pollen count.