People in dense-urban areas already spend around 90% of their time indoors between their homes and workplaces. The stay-at-home measures we have implemented due to the Coronavirus pandemic have brought the time spent indoors closer to 100%. Although these measures have been proven to slow down the spread of Coronavirus, do they mean that confined space, such as an apartment, is COVID-proof?
We tend to think of HVAC (Heating Ventilation and Air Conditioning) systems as a way to either heat the room in winter or to cool it during the summer. A well-designed HVAC system should do more than just bring comfort to a home; it should contribute to healthy Indoor Air Quality (IAQ).
COVID-19 aside, ~50% of all illnesses are caused or aggravated by poor Indoor Air Quality. How do air circulation and poor maintenance of HVAC systems in buildings contribute to poor IAQ? In dense, urban areas, do fossil-fuel burning systems bring outdoor pollutants and unwanted contaminants inside buildings? How does residential construction contribute to respiratory illnesses that are related to COVID-19?
This document explores and begins to explain air quality and environmental health measures and upgrades that multifamily buildings owners in dense urban areas should implement to improve health during the COVID-19 crises and beyond. Starting with scientific facts and principles, this document provides an inventory of building ventilation designs, ventilation design challenges, and ends with strategies and best practices for increasing IAQ and providing a healthy living environment.
Coronaviruses are very common: they have so far emerged in the form of the common cold (30% of cases), SARS, and MERS. The problem with coronaviruses is that they mutate very quickly and easily, which is part of what led to COVID-19 spreading so rapidly. This type of virus becomes airborne when an infected person expels respiratory droplets through coughing, breathing, and sneezing. Large droplets can remain airborne for thirty minutes and may infect people within a six-foot range. Smaller, lighter droplets can travel further and remain in the air for even longer.
In an apartment building, an infected person can continually replenish the airborne coronavirus by breathing, coughing, or sneezing in an apartment, but also in hallways and common areas. The risk of propagation can increase based on ambient factors, such as temperature and humidity levels. Social distancing, sufficient ventilation, maintaining recommended relative humidity levels, and surface cleaning can decrease the risk of contracting COVID-19.
According to Elizabeth McGraw, director of the Center for Infectious Disease Dynamics at Pennsylvania State University: "What we know is that they [the droplets] are better at staying afloat when the air is cold and dry. When the air is humid and warm, [the droplets] fall to the ground more quickly, and it makes transmission harder." The introduction of cold and dry air in the winter escalates the spread of Coronavirus. However, warmer temperatures will not make it disappear. Coronavirus can only be inactivated if exposed to temperatures above 133°F for 30 minutes or more, which is not likely to occur with residential air distribution systems.
No, “letting the sunshine in” will not kill Coronavirus and make an apartment safe because the sun's irradiation temperature at the surface of the Earth cannot reach 133°F. Furthermore, and although UV-C emitted by the sun would sanitize surfaces, the ozone layer in the atmosphere filters them. The UV rays that reach the surface of the Earth are UV-A and UV-B, which don’t have the disinfection properties of UV-C rays.
Understanding air movement in buildings is essential when investigating indoor air quality. Airflow patterns in a multifamily building are the result of combined forces that are dominated by stack effect (also called chimney effect) and by mechanical ventilation. McGuire (1967) emphasizes that “among these effects, the dominant natural influence on building air movement is the chimney effect but all of these forces must be considered and sometimes one or more of these can become the dominant factor in how air is moving indoors”. Indoor air movement and conditions are affected by changes in wind velocity, wind direction, and outdoor temperatures.
Following the second law of thermodynamics, air moves from higher pressure levels towards lower pressure levels through paths of least resistance (holes in walls, plumbing penetrations, shafts, etc.) carrying odors, humidity, bacterias, fungi, and viruses along. Unless a building is properly air sealed via compartmentalization, air could very well move from one apartment to another, increasing the risk of cross-contamination. Human activities such as walking, breathing, or using equipment also contribute to air movement within the living space.
Dilution ventilation, or mixing ventilation, is a traditional method for supplying conditioned air to spaces. Conditioned air is blown in through the ceiling or wall and dilutes the air in the room in an attempt to provide an even temperature but, unfortunately, it also evenly distributes contaminants through the space. Opening a window is an example of dilution ventilation.
With displacement ventilation, air is introduced at low velocity into a room at a low level and is exhausted at the ceiling. The flow of air is maintained by convective forces, which also have the effect of the concentration of pollutants rising from floor to ceiling. Displacement ventilation leads to cleaner air, as contaminants are removed from the occupied zone in a room towards the ceiling, as well as fewer complaints about drafts. Displacement ventilation is used mostly in Europe, and rarely in residential applications.
Controlling the humidity levels in a living space greatly influences comfort, energy savings, and reduces the risk of pathogens growth. A home will feel most comfortable at percent relative humidity (%RH) levels between 30% and 50%, but what does it mean for IAQ?
Highlighted in blue, the “optimum zone” in the graph below shows that the lowest amount of contaminants occurs when relative humidity is kept between 40% and 60%, with optimal results at 50% RH. Not only viruses, but also bacteria, fungi (e.g. mold), and mites have a hard time proliferating in that zone. 50% RH is also the best value for decreasing the risk of asthma or respiratory infections.
The American Lung Association published a “state of the air” report card system that lists, by state and county, levels of ozone and particle pollution and what groups of people are the most at risk. Data shows that these levels are much higher in dense-urban areas than in low-density suburbs, and it becomes clear that multifamily building occupants who open their windows to bring in “fresh air” are also bringing in pollutants that may affect their health. HVAC systems that bring outdoor air to a building ventilation system without cleaning it also put occupants at risk. Studies have found exposure to air pollution creates long-term health issues and there is a clear correlation between exposure to atmospheric particulate matter that have a diameter of less than 2.5 micrometers (PM2.5) and increased mortality risk, especially for respiratory and cardiovascular causes. In April 2020, a study released by Harvard University showed that “an increase of 1 μg/m3 in PM2.5 is associated with an 8% increase in the COVID-19 death rate (95% confidence interval [CI]: 2%, 15%)”. While the main source of air pollution is attributed to vehicles passing by a building, fossil-fuel burning heating systems such as boilers and domestic hot water makers rank second.
Unless a building was built after 1973, there is little chance that it was designed with IAQ in mind. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) develops ventilation standards, and published the first version of Standard 62 in 1973 to “specify minimum ventilation rates and other measures for new and existing buildings”. The initial goal and intent of these standards was to provide better IAQ to building occupants and reduce risks of adverse health effects (source: ASHRAE). ASHRAE editions 62.1 and 62.2 (for low rise residential buildings) have since been regularly updated.
In dense urban areas like New York City, Boston, or Chicago, most multifamily buildings were built before 1973, with no IAQ design in mind. Most of those buildings use natural ventilation to allow outdoor air in and stale air out without mechanical systems. Advanced sustainable design projects are recently being built with natural ventilation at the core of the ventilation system. The advanced sustainable design projects require, however, complex architectural design and buildings located outside of continental climates zones (mild summers, mild winters). In the existing built environment, especially in buildings that are over 50 years old, natural ventilation provides no guarantee that stale air evacuates the building quickly and efficiently.
Exhaust-only ventilation systems, or spot ventilation, remove stale air from buildings. These systems consist of vertical ventilation shafts that connect to apartment “wet rooms”, like bathrooms and kitchens. At the end of each shaft, a roof exhaust fan extracts stale air, either continuously at a fixed flow rate, or at specific times of the day if a timer is installed to control the exhaust fan operation. Because this system creates negative pressure, fresh air enters the apartments via paths of least resistance: open windows or gaps and holes in exterior walls. How and where air travels from those openings through apartments and to the exhaust shafts depends on the air pressure differential between zones. Air can also be pulled out of hallways, plumbing shafts, and ultimately other apartments. Thermal discomfort and unacceptable indoor air quality are particularly likely when occupants keep their windows closed because of extreme hot or cold temperatures. Problems related to under ventilation are also possible when infiltration forces such as wind speed and atmospheric pressure differential are weakest when no heating and no cooling are necessary.
As basic as these exhaust-only ventilation systems appear to be, they require proper design to ensure that the right amount of stale air is exhausted from the apartment, based on cubic feet per minute (cfm). A ventilation shaft should be designed with smaller sections on the top floor, which is closer to the roof fan, and larger sections at the bottom floors. Another important matter is the maintenance of these systems: fans that are belt-driven must be inspected regularly to ensure that the belt is not broken and that their motor is operational. When one exhaust fan fails, stale air is not extracted from the wet rooms connected to the shaft, and can also travel to other spaces in the building. In addition, exhaust shafts collect dust, grease, moisture, and other pollutants with time. They must be cleaned regularly to ensure proper extraction of air.
In smaller multifamily buildings, ductwork is typically the means of travel for conditioned air. Over time, these air ducts collect airborne particles and contaminants requiring regular professional duct cleaning. The majority of people don’t clean their duct systems, which allows particles to build up. Breathing in these particles could aggravate existing conditions such as asthma and allergies, and can spread illnesses around living spaces.
Newly constructed multi-family buildings are designed with advanced ventilation systems that can ensure proper balancing, meaning that volume of outside air supplied to a space equals the amount of air exhausted from that space. Additional features include heat or energy recovery to maintain low HVAC energy consumption.
As humans, we require fresh air to feel alive. Opening a window for a few minutes certainly doesn’t hurt, but leaving windows open would increase energy consumption because the HVAC system would have to work harder to maintain a set temperature. So how is the amount of fresh air controlled in most multifamily buildings?
As more people are added to a space, the level of CO2, and therefore the need for fresh air, increases. Except in commercial applications like auditoriums, meeting rooms, or schools, where occupancy can vary greatly, ventilation systems in multifamily buildings are designed and sized at a fixed air exchange rate. Therefore, the only means of bringing fresh air into most multifamily buildings is opening a window, which would put occupants at risk if levels of PM2.5 are high.
Central heating systems in multifamily buildings typically use steam or hot water (hydronic) to distribute heat to either fan coil units, baseboards or radiators. Fan coil units may be equipped with rudimentary filters that would catch large particulates like pet fur, or dust. However, no filtering is possible when heating terminals are baseboards or radiators. This is particularly inconvenient during winter when air could be dry and pathogens abundant.
Cooling systems have one advantage over heating systems when it comes to filtering because they circulate air and therefore filters can be installed in the air stream to stop some pollutants. Most fan coil units and window air conditioners have filters that can only stop large particulates.
Central forced air systems that use air handlers can be found in very large and relatively newer multifamily buildings. These are the same systems used in commercial buildings, and offer the best options for filtering air at the supply, return, or both. Air filtering, air purification, and anti-bacterial filtering options are available with such systems, making them a great “protective shield” against the spread of airborne pathogens like bacteria, viruses, or fungi. Unfortunately, upgrading a multifamily building to such a forced-air system would be technically challenging and certainly cost-prohibitive.
Relative humidity within a residential space should be kept between 40% and 60% to optimize comfort and to reduce the risk of spreading pathogens. According to Dr. Stephanie Taylor, international consultant and core member of the newly formed ASHRAE Epidemic Task Force, moisture control has a higher impact on limiting virus spread than trying to implement a 100% outdoor air solution, which may result in balancing problems.
Unless a building is equipped with central air handling systems that provide both humidifying and dehumidifying controls, there is currently no means for an HVAC system to control humidity levels at a specific setpoint. Separate humidifier devices would have to be installed in the building. Consequently, multifamily building occupants may be at risk during the winter in climate zones where outdoor air can be too dry, and also at risk during summer if air conditioning is not provided, forcing relative humidity levels to remain over 60% for an extended period of time. Fortunately in the winter, normal human activities like breathing, cooking, or bathing add moisture to the outdoor air and help maintain relative humidity in an acceptable and safe range. However, in the summer, humidity control is essential, and can be achieved by operating properly sized air conditioning units that dehumidify as they cool a space. Note that oversized air conditioning systems would have shorter cycles than properly sized systems; because they will turn on and off more often, less moisture will be evacuated from the living space.
Is the six-foot distancing rule enough to avoid contagion? Larger droplets coming out of someone’s mouth while talking, coughing, or sneezing would certainly travel less than 6 feet before dropping to the floor. Lighter droplets are a concern because an outdoor breeze may carry virus-laden breath farther than six feet and contaminate a person, since they can travel much farther and stay airborne much longer than larger droplets.
Indoors, heating and air conditioning units may also draw virus-laden air toward certain parts of a room based on the velocity and direction of airflow. Another concern is some HVAC systems can cycle out stale air containing viruses and other particles just like ceiling fans and other dilution ventilation equipment. In large commercial buildings equipped with central ventilation systems where recirculation is used, the risk of cross contamination is high. One possible route for cross contamination is the passage of airborne dust via the ducting system when the HVAC system is off or in a failed state. Unless equipped with appropriate filtering, systems that use dilution (or mixing) ventilation would increase the risk of virus spread.
A displacement ventilation system (see below) would significantly reduce the risk of propagation as low air velocity would force droplets to quickly fall on the floor. While this system is a great option for new construction, its implementation would be very costly as a retrofit.
A vast majority of portable air purifiers will only capture particles 0.3 microns or larger. Viruses are roughly 100 times smaller than bacteria and typically range from 0.004 to 0.1 microns in size. Therefore, a basic air purifier is unlikely to capture COVID-19 or other viruses.
The previous section shows that multifamily buildings have rather simple HVAC designs compared to more complex and often much larger commercial buildings. In the latter case, central ventilation systems could be upgraded, at relatively low cost, with advanced filtering systems that would eliminate the spread of pathogens in the air distribution systems. In contrast, a large majority of multifamily buildings lack even the most basic air filtering equipment, because their HVAC systems are based on steam or hot water circulation and fresh air cannot be mechanically supplied to these living spaces.
Air sealing helps reduce uncontrolled air movements in a building. An air sealing strategy should be implemented along with ensuring that the minimum ventilation requirements are met. This measure also:
Examples of air sealing strategies include:
The ultimate goal of air sealing strategies is to compartmentalize the apartments in a building. Compartmentalization reduces the stack effect in a building and arranges for fresh air to be supplied from outdoors and not from other apartments or common areas. It also ensures that stale air is exhausted outside of the building without traveling to other spaces.
Scientists with Princeton University, the University of California-Los Angeles, and the National Institutes of Health (NIH) recently conducted a study that found “viable viruses could be detected in aerosols up to 3 hours post aerosolization.” Because COVID-19 is spread mainly from person-to-person through respiratory droplets produced when an infected person coughs or sneezes, the Center for Disease Control recommends increasing building ventilation as a justifiable measure to cut down on recycled contaminated air.
Buildings equipped with a central air system could, for instance, keep minimum outside air damper positions at 20% or higher, or run HVAC fans continuously at design speed to maintain high air changes, but these measures are not applicable for older buildings constructed prior to the first ventilation code and therefore lack outside air intake and exhaust. In the latter case, which includes most multifamily buildings, the following measures should be considered:
As mentioned in the “When Outdoor Air is Not That Fresh” section, dense-urban areas are seeing growing outdoor air pollution that is exacerbating people’s health conditions. While bacteria, viruses, and fungi would likely not be coming from the outside, other pathogens contained in the air surrounding a building may affect people’s health. Therefore, it is important to consider measures that would increase air filtering and decontamination. The COVID-19 situation should be the catalyst for implementing air treatment and comprehensive IAQ solutions in multifamily buildings.
The following proposed measures include air treatment solutions that have been proven in buildings other than multifamily residential:
Unless a central air distribution system is supplying a building, there is no means for maintaining “safe” humidity levels between 40% and 60% RH in a living space other than to install portable evaporative humidifiers in each apartment. The cost for these devices ranges from $100.00 to $700.00.
These portable humidifiers are required to be filled up frequently, sometimes more than once per day since they don’t connect to a water pipe. This manual intervention may burden occupants in the rental sector, unless they understand the benefits of operating this device.
Most commercially available air purifiers or filtering solutions are high-efficiency particulate air (HEPA) filters. A typical HEPA filter can remove many particles from the air but is not going to remove coronavirus because, like many other viruses, it is too small: a HEPA filter is 0.3 microns while the virus is about 0.1 microns.
An anti-bacterial filter removes particulates and harmful pathogens like viruses from the air before recirculating it into rooms. Anti-bacterial filters are generally installed as a complement to standard filters to trap dust particles and kill microorganisms by using positively charged elements to attract negatively charged particles. These filters strain particles through microscopic pores and fibers, thus effectively trapping dust and other pollutants. By removing dust particles, these filters also extend the life cycle of air conditioning units and improve their performance.
One disadvantage of the anti-bacterial filter is its short lifespan. An anti-bacterial filter can last for approximately 3 to 6 months. Although filters could be cleaned and washed, scheduling their replacement at regular intervals is the best option.
Sunlight contains three types of ultraviolet rays: UV-A, UV-B, and UV-C. All UV types can be harmful to humans as they are capable of penetrating deep into the skin and damaging DNA. UV-C, which consists of short and energetic wavelengths of light, is capable of destroying viral particles like viruses, but can however be very dangerous to humans if exposed directly to people's skin. Since the late 1800s artificially generated UV-C has been used for sterilization purposes in hospitals, offices, and factories. Today, manufacturers offer UV-C technologies in commercial air handling units and rooftop units to enhance indoor air quality, either as an embedded system or as a retrofit kit.
There are however fewer applications for the residential sector, though several manufacturers offer UV-C retrofit kits that are now available on the market. Whether they are installed in-duct of a central air conditioner, in a Packaged Terminal Air Conditioning (PTAC) unit, or in the indoor unit of a heat pump system, UV-C retrofit kits can be very effective. These UV-C emitters must be designed with high energy output to properly kill viruses and other pathogens.
Other applications include the integration of UV-C kits in air purifiers, or in professional hand-held devices. About 6 seconds of direct exposure to ultraviolet germicidal radiation is sufficient to decontaminate a surface.
One disadvantage of UV-C equipment is that the lamp consumes electricity (about 50W to 75W) and must be replaced every 6 to 12 months, based on operating time.
Multifamily buildings in dense urban areas are generally equipped with a central boiler (steam or hot water) with piping feeding radiators or baseboards that do not offer air filtering capabilities. With this configuration, only portable equipment could provide the air filtering needed to treat living areas. New air-source heat pumps (ASHP) are an alternative to traditional heating systems. These electric systems are capable of supplying heating and cooling very efficiently even in cold climate zones and have air filtering and dehumidification capabilities. Air transfers between rooms are reduced, thus limiting the risk of pathogen contamination.Therefore the conversion of steam or hot water systems to air source heat pumps would increase IAQ.
Furthermore, ductless systems could offer multistage filtration that consists of a standard air filter to stop dust and larger particulates, with the addition of an anti-bacterial filter that can eliminate up to 99.9 percent of bacteria and viruses, for optimal improvement of indoor air quality.
An alternative to anti-bacterial filters would be to equip or retrofit ASHP indoor units with a UV-C light kit that would provide similar results, though consuming more energy to operate.
Like a room air conditioner, a properly sized ASHP will dehumidify the air in cooling mode, keeping relative humidity below 60% and thus reducing the risk of pathogen propagation.
Whether it is to make people aware of preventative measures to contain the spread of a virus such as COVID-19 or to share general information about improving air quality, multifamily building owners and building management companies should consider implementing effective tools to communicate with the residents. During the COVID-19 pandemic, TV channels and the Internet provided ample information about the situation. However, the information provided by different media was at times conflicting and could change multiple times a day. Building residents would likely feel more comfortable if clear instructions came directly from the building they live in: consistent messaging, one source of information.
In a stay-at-home or no-contact situation, the most effective tools would be written communication like emails, memos, or poster boards that can be posted in the lobby and/or on each floor of the building (e.g. elevator landings). Other advanced communication techniques may include webinars or recorded video messages. The use of visuals like photographs, charts, or graphs would provide helpful context alongside written communication. Topics may include:
The COVID-19 crisis has created new social rules on how we approach and protect ourselves from global health threats. In the past 3 months, thanks to the Internet and TV media, we have collectively acquired immense knowledge about pathogens, how they spread in our environment, and how to mitigate the risk of propagation. We also learned that our living and working environments have deficiencies and that improving indoor air quality should become one of our highest priorities.
Multifamily buildings in dense-urban areas were, for a large majority, erected before the first set of ventilation standards was released in 1973, and therefore have no IAQ design features. In those buildings, not only is indoor air prone to the risk of pathogen proliferation, but outdoor air that enters buildings is polluted by exhausts from vehicle traffic and other sources of fossil-fuel burning equipment like boilers. Thus, opening a window to allow “fresh air” to dilute with indoor air is not ultimately a solution.
Advanced filtering solutions such as anti-bacterial filters and UV-C light disinfection have been used effectively in hospitals for decades to kill bacteria, viruses, and fungi. These technologies could be easily implemented in multifamily buildings that use air circulation for heating and cooling. Central and individual air conditioners, but also air source heat pumps, could adopt these technologies to improve air quality. Along with these measures, proper air sealing to avoid cross-contamination between apartments and keeping humidity levels between 40% and 60% relative humidity has proven to be effective in limiting the propagation of common pathogens present in buildings.
Finally, and in an effort to combat health threats and implications of the coronavirus pandemic, building owners must prepare to inform and educate their residents. Communication tools may include frequent awareness campaigns and guidance on how to prevent and respond to a pandemic. Best results occur when a building owner or management company clearly explains how they plan to implement best practices for increasing global environmental health. Engineers and scientists have the ability to design and upgrade building systems for higher indoor air quality; however, technical improvements must be implemented alongside clear communication tools.
The COVID-19 pandemic has been an eye-opener about health disparities in dense-urban communities. With a greater focus on implementing IAQ measures, people in multifamily residential buildings will live healthier lives and be less at risk during pandemics.
1. The American College of Allergy, Asthma & Immunology. Global Indoor Health Network: https://www.survivingmold.com/docs/GIHN_STATEMENT_2_2012.PDF
2. Chimney effect, where warm air rises and cool air falls, may be increased by building shapes, natural chimneys such as stairwells or elevator shafts, and building exhaust ventilation systems.
3. McGuire, J. H. "Smoke movement in buildings." Fire Technology 3, no. 3 (1967): 163-174.
4. Exposure to air pollution and COVID-19 mortality in the United States: A nationwide cross-sectional study (Updated April 24, 2020) Xiao Wu MS, Rachel C. Nethery PhD, M. Benjamin Sabath MA, Danielle Braun PhD, Francesca Dominici PhD. All authors are part of the Department of Biostatistics, Harvard T.H. Chan School of Public Health, Boston, MA, 02115, USA