Ventilation
Workplaces need to be adequately ventilated. Fresh, clean air should be drawn from a source outside the workplace, uncontaminated by discharges from flues, chimneys or other process outlets, and be circulated through the workrooms.
Ventilation should also remove and dilute warm, humid air and provide air movement which gives a sense of freshness without causing a draught.
If the workplace contains process or heating equipment or other sources of dust, fumes or vapours, more fresh air will be needed to provide adequate ventilation. Windows or other openings may provide sufficient ventilation but, where necessary, mechanical ventilation systems should be provided and regularly maintained.
Source:
http://www.hse.gov.uk/pubns/indg244.pdf