HVAC systems provide comfort and quality air for patients, staff, and visitors in hospitals. Comfort is generally determined by temperature, humidity, and air speed. Air quality is generally defined by particle count — both organic and inorganic. Current design codes and standards specify precise levels of filtration, air change rates, and outside air for dilution, and vary for each space in a hospital. These are the minimum standards used by engineers to design hospitals. Of course, over time the performance of the HVAC is determined primarily by the operation and maintenance of the systems and the activities of the occupants.