Ventilation

Simply put, ventilation is a system that provides outdoor air to a building. There are two types of ventilation: mechanical ventilation actively moves outdoor air into a building, and natural ventilation relies on the passive flow of air from outdoor to indoor spaces.  Ventilation moves outdoor air into a building to replace any pollutants that might have accumulated in indoor air and is thus an integral aspect of indoor air quality (IAQ). In mechanically ventilated buildings, the supply air coming out of an air diffuser or other supply vent is typically a mixture of outdoor air and air that has recirculated through the building. Naturally ventilated buildings are designed and built in a way that relies on natural pressure differences to move external air to an indoor space. Most homes are said to be naturally ventilated, in that outdoor air comes indoors through open doors and windows and because most homes are "leaky." Heating and/or cooling systems in homes typically recirculate the same air after treating (heating or cooling) and are thus not part of the official ventilation system since they are not bringing in outdoor air.

Ventilation can impact indoor air quality and health

Ventilation is an important control strategy for maintaining good IAQ and improving poor IAQ. Properly designed ventilation can dilute particles and gases in indoor air and prevent contaminants from accumulating to levels that may cause health or comfort problems. It can also remove excessive moisture and prevent microbial growth.

Ventilation should be used in conjunction with source control, by preventing contaminants from entering the air, and, in some cases, air cleaning (see Summary of Available Information on Residential Air Cleaners from U.S. EPA ). In general, it is more efficient to eliminate or reduce sources of indoor pollutants or use local exhaust ventilation or air cleaning devices to remove them near their source than to control them by general ventilation. For instance, exhaust fans over kitchen stoves and in bathrooms are effective ways to remove combustion products and excess moisture, respectively, from those point sources.

Ventilation rates that are too low can cause pollutants and moisture to build up to levels that may lead to unpleasant odors, discomfort, or even adverse health effects. Lower ventilation rates have been associated with higher symptom rates and reduced performance in school or work settings. Therefore, higher ventilation rates are generally desirable from the IAQ point of view. However, there are two reasons not to provide exceedingly high ventilation levels:
1) Increasing the ventilation rate may increase energy consumption if the outdoor or recirculated air must be "conditioned", that is, cooled or heated and perhaps dehumidified or humidified.
2) If a contaminant is present at higher levels in the outdoor air that is brought into a building (for example, ozone or pollutants exhausted from vehicles), then increasing the ventilation rate may bring in pollutants more harmful than those that are being removed, unless the air coming in is treated to remove the contaminants from outdoors.

In practice, building ventilation rates need to balance energy consumption with the known or expected health and comfort benefits. More information regarding the effect of ventilation rate on IAQ, health, and productivity can be found at the website of Lawrence Berkeley National Laboratory's Indoor Environment Department.

Ventilation Codes

Standards and codes establish minimum ventilation rates to provide comfortable indoor environments and protect human health. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends minimum ventilation rates for different types of buildings and spaces (known as ASHRAE Standards 62.1 and 62.2), which are often incorporated into national, state, and local codes. Typically standards and codes specify the minimum ventilation rate per person or per unit of floor area. The minimum rate varies with the type of building or usage of the space; for example, a higher air change rate is recommended for health club weight rooms (26 cfm/person based on default occupant density) than for office spaces (17 cfm/person) because the higher activity levels in the weight room will produce more occupant-related odors. The Code of Regulations identifies the minimum ventilation rates required for California buildings; see California's Energy Efficiency Standards for Residential and Nonresidential Buildings.

California workers are fortunate in that not only must workplace mechanical ventilation systems be designed and built to meet code requirements, the systems  must also: (i) be operated to provide at least the quantity of outdoor air required by code, (ii) be operated continuously during working hours (with exceptions), (iii) be inspected at least annually and problems corrected within a reasonable time, (iv) be maintained, and (v) records of all system inspections and maintenance be kept in writing and available within 48 hours of request (see the Cal/OSHA Minimum Ventilation Standard).