Ventilation can do one of two things to improve or maintain indoor air quality. It can replace contaminated indoor air with new and fresh outside air, or it can dilute indoor air, to ensure indoor air quality is maintained. Poor ventilation may make people feel sick. Generally the poor ventilation complaints may include:

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• dry skin
• headache
• nasal congestion
• fatigue
• difficulty breathing
• itchy eyes
• dry throat.

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These symptoms are sometimes known as sick building syndrome. The onset of sick building syndrome can cause the following general symptoms:

• People experience colds, allergies, fatigue, flu-like symptoms,
• Symptoms are widespread,
• Symptoms disappear when people leave the building, and
• Onset is sudden and usually follows a "change" such as painting or new carpets.

Indoors

Carbon dioxide level is often used as an indicator of indoor air quality. The lower the level of carbon dioxide, the better the quality of air.

• Carbon dioxide level in the outside air is 350 ppm.
• Carbon dioxide level indoors must not exceed 700 ppm above outdoor level.
• In a well ventilated area, the carbon dioxide level is 500 to 900 ppm.

Ventilation rate depends on the type of activity. If toxic gases and vapours are produced indoors, the ventilation rate should be sufficient to remove these contaminants and keep the level within acceptable limits.

Ventilation Principles

During any classroom activity involving the use of chemicals, increased air circulation or ventilation is necessary to add clean air with enough oxygen to the space and to remove contaminated air. Ventilation also controls the potential for fire and explosions and, helps keep the temperature and humidity at a comfortable level. There are two basic ways that chemicals are removed from the air around us:

• Dilution ventilation, and
• Local exhaust (fume hood).

General (Dilution) Ventilation is the ventilation used inside homes, offices or classrooms. Intake and exhaust vents are generally on the walls and ceiling. This type of ventilation allows fresh air to be added to air in a room, and helps to "freshen" the air. However, it can also help spread chemical vapours around rather than removing them directly. General ventilation works best in offices, homes, schools, etc. where no hazardous chemicals may be present.

Local Exhaust Ventilation is the movement of air from a specific, small area. This is the type of ventilation used in home bathrooms or above stovetops (usually fans pushing air out of the house). In a chemical laboratory, chemical fume hoods have fans which pull the air through the hood face and exit into an exhaust system in the wall.

All fast food kitchens will have local exhaust ventilation over the fryers and cooktops to eliminate smoke from the cooking process. In woodworking shops some pieces of equipment may have an exhaust ventilation system which removes wood dust generated during the cutting process.

When working with hazardous materials the MSDS will have specific recommendations for ventilation.

In Canada, requirements for ventilation are usually specified in the Building Code (which varies from province to province). These requirements must be taken into consideration during design, renovation and maintenance of the building. ASHRAE Standard 62 of the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) is generally used as a guideline for ventilation design.