Stale air
Stale air (sometimes called vitiated air) is air that has been affected by human, animal, mechanical or other activities, the effect of which requires that the air is replaced with fresh air to ensure optimal conditions. This might be the result of perspiration, smoking, cooking, use of appliances, musky carpets and fabrics, lack of ventilation and so on.
In their calculations for air extraction, services engineers may consider specific volumes of stale air in m2 to be extracted per hour. They typically work to achieve desirable minimum fresh-air requirements for people and these will vary according to the activities concerned. These ventilation rates or air-change rates determine how many times per hour the stale air in a space should be completely replaced by fresh air.
Approximate air-change rates per hour for various accommodations:
| Accommodation | Air-change rates per hour |
| Factories (large, open) | 1-4 |
| Classrooms and libraries | 2-4 |
| Offices (above ground) | 2-6 |
| Public lavatories | 6-12 |
| Restaurants | 10-15 |
| Kitchens | 20-60 |
Air change rates may need adjustment according to specific conditions that might prevail eg whether smoking will take place or the presence of other fumes. Also, higher levels of relative humidity (ie above 70%) may require higher ventilation rates to reduce the water vapour content in the air.
For more information see: Air change rates.
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