While many of us spend more time indoors than ever before, the pollution levels of indoor air are two to ten times higher than those found outdoors. Indoor air quality (IAQ) can be compromised by sources such as building materials, fuel-burning appliances, household cleaning and maintenance products, heating and cooling systems, tobacco products, and excess humidity. Many of these emit volatile organic compounds (VOCs) into the air by off-gassing and evaporation. Though there are several strategies for reducing VOCs and other pollutants in indoor air, including source control and air filtration systems—the information that follows in this article will focus on the role of ventilation.
A well-ventilated indoor space is one in which occupants are healthier and happier. Ventilation removes VOCs from buildings, resulting in lower rates of airborne infectious diseases and health effects, and fewer symptoms of Sick Building Syndrome (anything from headaches and eye, nose, and throat irritation to fatigue, nausea, and respiratory ailments). Studies in schools show that in well-ventilated classrooms, attendance is higher, and academic performance is too—students are faster and make fewer errors. A joint study by Harvard, SUNY Upstate Medical University, and Syracuse University showed similar results in the workplace: people working in a building with low VOCs and good ventilation experienced huge jumps in their cognitive functions, particularly in areas considered “high-level,” including crisis response, strategy, and information usage. Poor IAQ disproportionately affects vulnerable populations, like children, the elderly, those with chronic conditions, low-income, and minorities.
Throughout almost the entire history of built indoor spaces, outdoor air has entered directly into buildings, but with the advent of electricity, many of today’s buildings depend on mechanical ventilation rather than natural airflow. The recent industry push for high-performance buildings that expend less energy on heating and cooling has led, in many cases, to more insulated, airtight buildings. A side effect has been reduced ventilation—indoor pollutants remain concentrated in the air, with nowhere to go. But even though they seem incompatible, with proper design and installation it is possible for a building to be both energy efficient and have good ventilation.
How Does Ventilation Occur?
There are three main ways for outdoor air to enter and leave a building: natural ventilation, infiltration, and mechanical ventilation. Natural ventilation is the movement of air through open windows or doors. Along with window shading, it can also be used to regulate indoor air temperature, which helps to moderate the use of heating and cooling systems as well as reducing the IAQ effects of these systems. Infiltration is when air enters through openings, joints, and cracks in walls, floors, and ceilings, and around windows and doors. It happens in all buildings to some extent. Natural ventilation and infiltration are both passive methods, in which air movement occurs through temperature and pressure differences between indoors and outdoors, and by wind.
Finally, mechanical ventilation is an active method, which intakes and removes air from indoor space using heating, ventilation, and air conditioning (HVAC) systems. These range from outdoor vented fans that remove air from a specific room, like a bathroom or kitchen, to whole-house systems that remove indoor air and distribute fresh air throughout the house. These can be either intermittent or continuous. However, most heating and cooling systems don’t mechanically bring outdoor air indoors, though some newer systems are incorporating ventilation features, including energy efficient heat recovery ventilators that alleviate cooling and heating costs. There is a new market for air quality sensor technologies and dynamic response natural ventilation systems that work in tandem with air filtration systems.
The effectiveness of a ventilation system is dependent on how air is distributed throughout the indoor space. According to the air balance principle, air always flows from spaces with higher air quality to those with lower air quality. This means supplying air to the cleaner rooms, which then flows into more polluted rooms, where it is exhausted. In commercial buildings, outdoor air would enter into areas with high occupancy and be exhausted from rooms that generate pollution in order to maintain air balance (so that polluted air doesn’t flow into higher occupancy rooms). In residential buildings, air enters through bedrooms and living rooms, and is exhausted in bathrooms and kitchens.
The Outdoor Air Conundrum
Outdoor air enters a building through air intakes, whether mechanically—through HVAC system—or naturally ventilated—through windows. Conventional wisdom says that IAQ can be improved by bringing in outdoor air. But in cases where outdoor air is polluted too, this isn’t necessarily a great solution. Outdoor air contaminants include radon, pesticides, vehicle emissions, smoke, and refuse. It’s important to fully evaluate the ambient air quality and sources of contamination in the area where a building is, or will be, located. Designers and building professionals should be mindful of outdoor air quality when determining a building’s location and design in order to avoid negative health effects.
The most comprehensive ventilation standard in the US was established by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). This standard is used by various green building certifications and frameworks, including LEEDv4, Living Building Challenge, and Fitwel.