Indoor air is 2 to 5 times more polluted than outdoor air, according to the US Environmental Protection Agency, and we spend 90% of our time indoors. Building upgrades often focus on improving appearance, and energy retrofits have gained popularity in recent years. However, indoor air quality tends to get less attention, but it is fundamental for human health and productivity.
There are many ways to improve indoor air quality in buildings, but they normally follow three general strategies:
- Eliminating air pollutants at the source, by preventing the ingress of outdoor pollutants, while minimizing indoor emissions.
- Diluting air pollutants with ventilation systems, by constantly displacing indoor air with outdoor air.
- Removing air pollutants directly from the air, with methods like filtering and chemical purification.
These indoor air quality strategies are complementary, and they can achieve synergy when used together. HVAC engineers recommend minimizing the air pollution sources as a first step; this reduces the need for ventilation and air purification, which have an energy cost.
There are thousands of substances that can be considered air pollutants, and their effects vary depending on their concentration and composition. For example, off-gassing from new furniture can cause respiratory irritation, while carbon monoxide becomes life-threatening at concentrations below 1%. Carbon monoxide should not be confused with the carbon dioxide we exhale, which requires a much higher concentration to become dangerous.
Strategy 1: Eliminating Sources of Air Pollution
As mentioned above, there are many types of air pollutants. However, air quality indexes from around the world often include the six substances listed below. The US EPA designates these substances as “criteria air pollutants”, and their negative health impacts have been widely studied:
- Ground-level ozone, which is a respiratory irritant and a key component of smog. Ozone in the upper atmosphere protects us from solar radiation, but ground-level ozone is considered an air pollutant.
- Particulate matter or PM, which is classified into coarse PM (diameter below 10 microns) and fine PM (diameter below 2.5 microns). Fine PM is the most dangerous, since it can reach the inner lungs and bloodstream.
- Carbon monoxide, a combustion byproduct that is highly toxic for humans.
- Lead, which has been linked with health issues like kidney and brain damage.
- Sulfur dioxide, a combustion product and respiratory irritant.
- Nitrogen dioxide, also a combustion product and respiratory irritant.
Since many air pollutants are combustion by-products, appliances with a flame should be properly vented to release their flue gases outdoors. Indoor smoking must also be avoided, since cigar smoke contains large amounts of particulate matter and other pollutants.
Ozone sources are rare in building interiors. However, ionizing air purifiers produce ozone, and other purification methods are recommended for this reason. Ozone is also produced outdoors, through chemical reactions that occur when air pollutants are exposed to sunlight.
The effects of lead have been widely studied, and its use has been banned in many products as a result. However, lead can cause air quality issues in older buildings, especially if the walls were painted before lead-based paint was banned.
Volatile organic compounds (VOC) are not listed among the US EPA criteria air pollutants. However, they should be monitored closely when dealing with indoor spaces. VOCs are released by many construction materials and products commonly found indoors. Their effects range from respiratory irritation with brief exposure, to heart and lung disease with extended exposure.
Volatile organic compounds (VOC) can be deceiving, since many of them have pleasant odors. Many cleaning products and air fresheners have a high VOC content, and you might get the impression than the air is “clean”, when actually it is full of harmful chemicals. VOCs are also released by recently installed construction materials and new furniture. However, thanks to the popularity of green building certifications like LEED, the public is becoming more aware of VOCs and their harmful effects. As a result, many products are now available in low-VOC versions.
Strategy 2: Improving Air Quality with Effective Ventilation
Reducing the concentration of air pollutants to zero is impractical, but their harmful effects are eliminated at low levels. Institutions like the US EPA and World Health Organization (WHO) have determined exposure limits for common air pollutants, to prevent their negative health effects.
Ventilation improves air quality by diluting indoor air with cleaner outdoor air. Design guidelines are provided in the ASHRAE 62.1 Standard: Ventilation for Acceptable Indoor Air Quality. Two design approaches are presented for mechanical ventilation systems:
- The Ventilation Rate Procedure uses prescriptive ventilation rates that have been determined experimentally by ASHRAE. The ventilation rates calculated with this method change depending on the building type, floor area and occupancy.
- The Indoor Air Quality Procedure does not provide prescriptive ventilation rates. Instead, the ventilation designers must identify key air pollutants, and specify a ventilation system with enough capacity to keep them under control.
Both design approaches result in a ventilation system that meets the indoor air quality requirements from ASHRAE. For maximum performance, the ventilation system can use automatic controls and air quality sensors. When the sensors detect an increase in air pollution levels, the ventilation system can increase outdoor airflow to reduce their concentration.
Strategy 3: Removing Air Pollutants with Filtering and Purification
Air filters are a normal component of ventilation systems, and they capture many harmful particles as the air flows through. However, additional filtering devices can be used indoors, and there are also air purifiers that use methods like ultraviolet radiation and activated carbon. High-efficiency particulate air filters (HEPA) are highly effective, capturing 99.97% of particles with a diameter of 0.3 microns or greater.
Research from NASA also indicates that indoor plants can improve indoor air quality. Plants absorb several air pollutants as part of their normal metabolism, and they don’t have an energy cost like air purifiers. However, flowering plants should be avoided indoors, since pollen causes allergic reactions in some persons.
Air humidity is not considered an air pollutant itself, but poor humidity control can have negative effects on indoor air quality. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) recommends a relative humidity range of 30% to 60%.
- High humidity provides an ideal environment for mold, dust mites and bacteria. All these organisms have a negative effect on indoor air quality.
- Low humidity can irritate the respiratory system, eyes and skin. Airborne particles and viruses also stay in the air longer with low humidity, increasing the change of inhaling them.
In other words, controlling air humidity is important to conserve indoor air quality. Humidity by itself is not an air pollutant, but extremely dry or humid environments suffer from reduced air quality.
There are many design measures and technologies that improve indoor air quality in buildings. However, the basic approach consists of eliminating pollution sources and using an effective ventilation system. Indoor air quality can be improved further with additional filters, air purifiers and indoor plants.
To improve indoor air quality without an excessive energy cost, building owners should focus on eliminating pollution sources first. Ventilation, filtering and air purification all consume energy, but their workload is reduced when air pollution sources are minimized.
About the Author
Michael Tobias is the founder and principal of Nearby Engineers and New York Engineers, an Inc 5000 Fastest Growing Company in America. He leads a team of more than 30 mechanical, electrical, plumbing, and fire protection engineers from the company headquarters in New York City, and has led numerous projects in New York, New Jersey, Chicago, Pennsylvania, Connecticut, Florida, Maryland, and California, as well as Singapore and Malaysia. He specializes in sustainable building technology and is a member of the U.S. Green Building Council.