A building is an enclosed structure with a roof and walls. At UW, buildings vary widely in use – some hold classrooms while others are used for offices or labs.
Ventilation systems in buildings can be natural or mechanical. Natural ventilation involves opening windows and other strategic openings. Mechanical ventilation involves using fans to move air.
Temperature
A building’s temperature affects the ventilation rate. Air at higher temperatures is denser and will tend to fall or sink, while cooler air is less dense and will rise. These differences in molecular density create pressure gradients that push the air through openings. Wind, which is the result of global and local thermal phenomena, exacerbates these pressure gradients by blowing or sucking air.
The natural ventilation rates of buildings are determined by several factors, including temperature, wind speed, and the building’s thermal mass. These variables can also be influenced by the presence of contaminants in the outside air, such as ozone or pollutants exhausted from vehicles.
In naturally ventilated buildings, air is brought in through openings on the windward side and sucked out through openings on the leeward side. These openings can be shaped to minimize energy loss, but accepted design avoids partitions in a room that are oriented perpendicular to the airflow, as this promotes better mixing and ventilation.
Studies of schools and office buildings suggest that higher ventilation rates decrease absenteeism and performance problems. In one study, an increase in ventilation from 15 to 50 cfm (7.1 to 24 L/s) per person led to a 35% reduction in short term absences. In general, the higher the ventilation rate, the more positive the results, though a large variation exists in study findings.
Humidity
Humidity is the amount of water vapor in air and is measured by a hygrometer. The higher the humidity, the more moisture in the air and the lower its dry-bulb temperature. Air with a high relative humidity can cause mold and mildew growth, which releases spores into the air. This reduces indoor air quality and can lead to respiratory problems, allergies, and asthma.
Humid air can also cause damage to building materials. When wood is exposed to humid air, it expands and can warp or crack. It can also promote fungus and mold growth. Metals are more likely to rust in humid air. High humidity can also cause condensation on surfaces and cause stains. In addition, it can increase the rate of electrostatic discharges. These discharges can damage electronic devices.
Most people understand that opening a window can help to lower humidity in a home, especially on a cool breezy summer day. But if the windows are left open past the point when they provide this benefit, it can cause humidity to rise to uncomfortable levels.
Household activities like cooking, bathing, drying clothes, and peoples’ breathing are the primary sources of humidity in homes. If the thermal boundary is not properly sealed, or is leaky in many places, it can let humid outdoor air into a house and make it feel too stuffy and uncomfortable.
Dust
Dust is a mix of dry and loose particles from indoor and outdoor sources, such as pet fur and dead skin cells; fabric fibers from clothing, carpets and upholstery; soil, pollen, and decomposed insects; and paper debris. In some cases, a variety of harmful pollutants can also be present, such as mold, bacteria, viruses, microplastics from packaging and clothing, dander from animals and human beings, second-hand smoke, volatile organic compounds (VOCs) and traces of metals.
Most people are aware that dust significantly impacts respiratory health. It irritates the nose, mouth and lungs of humans and can cause symptoms including itching, congestion, sneezing and coughing. Fine dust particles can penetrate deeper into the lungs and, in some cases, can even be absorbed into the bloodstream.
It is important for building occupants to follow a quality cleaning routine, such as regular vacuuming, to reduce dust accumulation in occupied spaces. Occupants should also avoid smoking and using household chemicals that emit VOCs, as well as allowing paints and sealants to off-gas outdoors before bringing them inside. In addition, buildings should have local exhaust ventilation to move dust that settles on upper surfaces down and away from the occupants.