Do I have a ventilation issue?
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1 - What types of ventilation systems are there?

Intentional delivering of outdoor air (ventilation) can be achieved either mechanically (that is, provided by the fan) or naturally (for example, directly from the outdoors through windows).  However, the design and operation of a ventilation system for homes may differ significantly from office, commercial, and institutional buildings.  Therefore, offices and residences are discussed separately below:
?    Office and commercial building ventilation
Mechanical ventilation systems are the most widely used in the United States for office and commercial buildings.  They often employ complex air handling systems that have outdoor air intakes and that use fans and ducts to distribute air throughout a building while exhausting some of the indoor air.  The air handler is typically equipped with a damper system that controls the amount of outdoor air delivered.  Depending on system design, the outdoor air damper position can be fixed (delivering a constant outdoor air flow rate) or varied based on occupant demand (referred to as demand control ventilation).  
The concept of natural ventilation has received more attention due to energy costs related to mechanically delivering and conditioning the outdoor air.  Natural ventilation relies on three physical mechanisms to exchange air between the outdoor and indoor spaces: wind driven, pressure driven, and stack driven.  Therefore, for natural ventilation to function properly it needs to be carefully designed (for example, correct size, orientation, and operation of the windows).  The design is based on a number of considerations (for example, local climate, wind conditions, location, and building characteristics).  An entirely natural ventilation system is generally more difficult and sometimes impractical for a larger building.
A hybrid system uses a combination of natural and mechanical ventilation.  It integrates the use of air-conditioning, when and where it is necessary, with the use of natural ventilation, whenever it is feasible or desirable.
?    Residential ventilation
Natural ventilation, by opening windows and doors, is the most common way to deliver fresh, outdoor air to homes.
Mechanical ventilation in homes and other smaller buildings can be achieved through a whole-house system or by spot ventilation.  
Whole-house ventilation (which has an active outdoor air intake that brings in outside air either continuously or at regular intervals) is uncommon in U.S. single-family homes.  However, such a system can more reliably provide the desired amount of ventilation air if designed properly and has gained attention as houses are built more airtight for energy conservation.  
The exhaust fans in a home bathroom or kitchen are usually operated as needed and provide spot ventilation if they are connected to the outside.  Outdoor air to replace this exhausted air enters the building through any windows or doors that are open, or through cracks in the building or around doors and windows.

2 - How do I know if I have a problem because of inadequate ventilation?

In homes, inadequate ventilation may result in excess moisture and odors, especially if local exhaust fans are not used in bathrooms and kitchens.  Excess indoor build-up of moisture can lead to mold, which creates odors and has harmful effects on health.  Inadequate ventilation may also result in accumulation of other pollutants emitted indoors.  For example, formaldehyde, which can cause irritation in the short term and in the long term is suspected to increase the risks of asthma attacks and cancer, can be emitted by building materials, cabinets, and other home furniture made of composite wood material.  Other volatile organic compounds (VOCs) may be emitted during smoking, renovation (for instance, from paint, polyurethane, caulk, or vinyl flooring), hobby activities (for instance, from glues, paints, or art supplies), cleaning products (for example, bleaches, ammonia, or oven cleaners), or pesticides.

In offices, many of the same pollutants can be produced indoors, and there is more likelihood of pollutants from office equipment.  Odors and symptoms of irritation may indicate too little ventilation ? research has shown that occupants of office buildings with less outdoor air ventilation experience more of a variety of symptoms, sometimes called sick building syndrome, that may include eye, nose, and throat irritation, headaches, or difficulty concentrating.  If you have these problems at work and they improve when you are not at work, it is possible that there is a problem at the workplace.  However, it is also true that offices that are too warm are associated with more symptoms, and occupants may perceive the air to be dry or stuffy even if there is sufficient ventilation.

In general, determining whether an IAQ problem is due to inadequate ventilation or other cause can be a complex question.

3 - What should I do if I have a problem?

For residences, the Homeowner?s Guide to Ventilation from the New York State Energy Research and Development Authority (NSERDA) website is a good guide.  It provides information on how to conduct a self-evaluation of your home?s ventilation system and how to properly select a contractor when necessary.  More information and resources regarding ventilation for homes are available at the USEPA website (http://www.epa.gov/iaq/homes/hip-ventilation.html).  
For large buildings (offices, schools, and so forth), ventilation systems are typically maintained by building facility managers.  USEPA guidelines are available for building owners and facility managers regarding how to operate and maintain building ventilation systems to achieve good IAQ (see http://www.epa.gov/iaq/largebldgs/#Building%20Air%20Quality).  The National Institute for Occupational Safety and Health (NIOSH) also provides information on building ventilation for better IAQ in workplaces (see http://198.246.98.21/niosh/topics/indoorenv/BuildingVentilation.html).

4 - What is infiltration?

Besides intentional natural ventilation, air inevitably enters buildings by ?infiltration,? which is the uncontrolled flow of air into an indoor space through gaps and cracks in walls.  (Think of the draft you may feel on a cold day from a poorly sealed door.)  Infiltration occurs in all types of buildings (homes as well as office buildings).  The amount of air that is exchanged between the indoors and outdoors through infiltration depends on a number of factors such as the size and location of the gaps and cracks in the shell, the strength and direction of the wind outside, and pressure and temperature differences between the building and outdoors.  Older buildings tend to have more infiltration unless they have been updated to be more air-tight.  Over the past few decades buildings have been constructed to be increasingly air-tight and have less infiltration to conserve energy.

5 - What is total air change rate?

The rate at which outdoor air replaces indoor air is described as the total ?air change rate,? which is the sum of mechanical ventilation, natural ventilation, and infiltration.  The air change rate is reported as air changes per hour, ACH, which is essentially the average number of times a volume of air equal to the entire inside volume of the building is exchanged between indoors and outdoors.  The air change rate equals the total mechanical ventilation rate when infiltration is negligible, as in a well-sealed or ?tight? building.  ACH is calculated by dividing the rate at which outside air is brought into a building (measured in cubic meters per hour, m3/h, or in cubic feet per minute, cfm) by the volume of the space being ventilated (measured in cubic meters, m3, or feet, ft3).  
The total air exchange rate can usually be measured by a ?tracer gas? method.  The tracer gas, such as sulfur hexafluoride (SF6) or perfluorocarbon tracers (PFTs), can be injected continuously or in a single burst into the supply air duct or released directly in the ventilated space.  The rate at which the tracer concentration declines is then monitored, from which the air change rate can be estimated.  
Another procedure, the blower door test, is commonly used to measure the air infiltration or leakage rate of a small building or home.  The blower door test measures the amount of air that flows into or out of the building while it is maintained at a set pressure difference between the indoors and outside.  By using a calibrated fan and metering equipment, airflow can be measured at a variety of pressure differences.  Blower door test results are standardized for an air pressure difference of 50 Pascal (Pa) and can be expressed in three ways: air changes per hour (ACH), airflow rate (m3/h or cfm), or equivalent leakage area (m2 or ft2).
Other techniques for measuring ventilation rates may include the use of a flow hood or an air velocity traverse at the outdoor air intake.