Building services include systems that make occupants comfortable, functional, efficient and safe, such as ventilation, air conditioning, water supply, drainage, plumbing and electrification. Ventilation involves supplying fresh air into a building and removing stale air. It is necessary to remove stale air, prevent concentration of odors and bacteria, and prevent buildup of flammable gases. Ventilation can be natural via openings, or mechanical using fans. Proper plumbing systems include water distribution, drainage, and sanitation to provide potable water and safely remove wastewater. Air conditioning cools, dehumidifies, filters and circulates air to maintain occupant health and comfort.