Stuffy homes are unhealthy homes, while homes with plenty of fresh air are healthy. That’s been a commonly held belief for at least 200 years. In the mid-19th century, the connection between ventilation and human health was championed by sanitarians, a group of health experts who blamed the spread of bubonic plague and cholera on “miasma.”
According to Michelle Murphy, the author of Sick Building Syndrome and the Problem of Uncertainty, “Ventilation engineers had previously promoted the mechanical supply of ‘fresh air’ in the name of healthfulness, not comfort. The fight against foul air, excess carbon dioxide, and miasma … had allied ventilation engineers with public health reformers, called sanitarians, who sought to improve the living conditions of the worthy laboring poor by … legislating standards for fresh air in tenements, schools, and factories.”
The miasma theory of contagion was disproved in the 1860s. However, the connection between ventilation and human health is still trumpeted by various organizations, including “healthy house” groups and fan manufacturers. For example, , a fan manufacturer, claim that “microbial pollutants like mold, pet dander and plant pollen along with chemicals such as radon and volatile organic compounds (VOCs) create a toxic environment in your home.”
Similarly, declares, “Ventilation is a critical component for home durability and occupant health.”
Since experts have posited a connection between mechanical ventilation in homes and human health for the last 160 years, perhaps it’s time to ask two questions:
The answer to the first question is no, not really. And the answer to the second question is an emphatic no.
When pigs fly
The standard for residential ventilation in the U.S. is ASHRAE 62.2. Among the members of the committee that developed ASHRAE 62.2 were Joseph Lstiburek, a principal of the Building Science Corporation, and Max Sherman, a senior scientist at Lawrence Berkeley National Laboratory.
To understand the range of expert opinion on residential ventilation, it’s a good idea to consult both Lstiburek and Sherman, since they rarely agree. For years, Lstiburek and Sherman have been arguing publicly about optimal ventilation rates. When it comes to almost every ventilation controversy, these two experts are usually at opposite ends of the spectrum. When I asked both experts about the connection between residential ventilation rates and human health, however, I was surprised to discover that they agree.
I sent an e-mail to Sherman asking him about available data on the connection between residential ventilation and human health, and he responded, “I think there is no data. We have simulations.”
When I asked Lstiburek the same question, he answered, “There is no data. To determine the health limits for residential ventilation, they just take occupational limits and divide them by 10 or 100. There is no science to it.”
The evidence is indirect
According to a useful Web resource from Lawrence Berkeley National Laboratory (LBNL), , “Very little research has been conducted on the relationship of ventilation rates in homes with the health of the occupants of the homes.”
Although researchers don’t have good data showing a link between residential ventilation rates and occupant heath, it’s possible to make inferences based on indirect evidence. The LBNL Web site notes, “From numerous experimental studies, as well as from theoretical modeling, we know that higher ventilation rates will reduce indoor concentrations of a broad range of indoor-generated air pollutants. Because exposures to some of these air pollutants, for example, environmental tobacco smoke and formaldehyde, have been linked with adverse health, we expect that increased home ventilation rates will reduce the associated health effects.”
This expectation has not yet been confirmed by researchers, however.
The three most frequently cited studies of the relationship between residential ventilation rates and occupant health were all conducted in Scandinavia.
One was a 1999 study by Norwegian researchers (Oie, L., et al., “Ventilation in homes and bronchial obstruction in young children,” Epidemiology, 1999, 10 (3), pages 294-299).
The second relevant study was a 2004 study by Swedish researchers (Emenius, G., et al., “Building characteristics, indoor air quality and recurrent wheezing in very young children (BAMSE),” Indoor Air, 2004, 14 (1), pages 34-42).
The third relevant study was a 2005 study by Swedish researchers (C. G. Bornehag, J. Sundell, L. HÃ¤gerhed-Engman, and T. Sigsgaard, “Association Between Ventilation Rates in 390 Swedish Homes and Allergic Symptoms in Children,” Indoor Air, 2005, 15 (4), pages 275-280).
According to the LBNL web site, the 1999 Norwegian study focused on young children. The web site notes, “Low home ventilation rates were not associated with an increase in bronchial obstruction (i.e., reduced breathing airflows) in children. However, the increase in risk of bronchial obstruction resulting from other factors, such as building dampness, was moderately to markedly higher in homes with ventilation rates below 0.5 ach. In other words, having low ventilation rates increased the health risks from some of the building conditions, such as dampness, that are associated with indoor pollutant emissions.”
According to the LBNL web site, the 2004 Swedish study “found that the risk of recurrent wheezing in children was not different for houses with measured air exchange rates above and below 0.5 ach.”
Like the two other Scandinavian studies, the 2005 Swedish study also focused on children’s health. The Swedish researchers found no association between residential ventilation rates and asthma rates in children. However, the researchers found that children with rhinitis and eczema had lower ventilation rates in their bedrooms than non-symptomatic children.
The authors of the 2005 study admit several limitations to their findings. They noted, “Residential factors not associated with ventilation (e.g., smoking, socio-economic status) may have impacted the findings.” Moreover, “the associations between ventilation rates and asthma and allergy symptoms were not strong, perhaps due to the small sample size.”
What’s the bottom line? According to the LBNL web site, “In summary, the few studies that have directly investigated whether lower ventilation rates in homes are associated with a worsening of health have had mixed findings.”
Factories and office buildings
We have somewhat more data on the effects of ventilation rates on human health in non-residential buildings (factories and office buildings) than we do in homes.
When it comes to office buildings, “The available data indicate that occupant health and perceived IAQ will usually be improved by avoiding ventilation rates below 20 cfm (9 L/s) per occupant and indicate that further improvements in health and perceived IAQ will sometimes result from higher ventilation rates up to 40 cfm (18 L/s) per person. These findings are relatively consistent for office buildings located in cold or moderate climates, but less certain for other building types and climates.” (ASHRAE Journal, August 2002: by Olli SeppÃ¤nen, William J. Fisk, and Mark J. Mendell.)
Three researchers — William J. Fisk, Anna G. Mirer, and Mark J. Mendell — studied the phenomenon known as “sick building syndrome.” The researchers reported, “Data from published studies were combined and analyzed to develop best-fit equations and curves quantifying the change in sick building syndrome (SBS) symptom prevalence in office workers with ventilation rate. … Based on these analyses, as the ventilation rate drops from 10 to 5 L/s-person, relative SBS symptom prevalence increases approximately 23% (12% to 32%), and as ventilation rate increases from 10 to 25 L/s-person, relative prevalence decreases approximately 29% (15% to 42%).” by William J. Fisk, Anna G. Mirer, and Mark J. Mendell.)
It is far from clear, however, that studies of office buildings or factories have much relevance for homes. , “We have some Threshold Limit Values (TLVs) for some occupational stuff. But that is occupational stuff. If you are factory worker and you are working with a particular chemical that we know a great deal about, NIOSH probably has an exposure limit for that particular chemical. But, TLVs only apply in factories (i.e. “occupational exposure”); they do not apply in office buildings and certainly not in houses. … I am an order-of-magnitude kind of a guy and I sometimes divide the occupational numbers by ten when pressed for an opinion for office and residential exposure. But this is an arbitrary guess on my part. There is no health data that I have to go on. Why not divide by a hundred? Some folks do. Are they more ‘right’ than me? They seem to think so. The point is that we are making this stuff up. All of us.”
Asthma and high humidity
Several researchers have shown a correlation between asthma symptoms in children and humidity problems in homes. According to William J. Fisk (“How IEQ Affects Health, Productivity,” ASHRAE Journal, May 2002), “Many studies have found that the prevalence of respiratory symptoms associated with asthma are increased by 20% to 100% among occupants of houses with moisture problems, implying that elimination of these moisture problems would diminish symptoms by 17% to 50% in these occupants.”
Most of these homes with serious moisture problems (for example, leaking roofs, plumbing leaks, or moldy walls) are substandard homes occupied by low-income families. While increased ventilation rates might be one way to tackle these problems, the homes may need more direct interventions instead — for example, roof repairs, plumbing repairs, or the installation of polyethylene on the crawl space floor.
Moreover, damp living conditions are only one factor associated with asthma symptoms. Other factors include the number of pets in the house and whether or not family members smoke tobacco.
Ventilating during hot, humid weather
When it comes to determining ventilation rates for U.S. homes, it makes sense to be wary of the conclusions of Scandinavian researchers — unless, of course, you live somewhere in the U.S. with a climate that resembles the climate of Sweden.
The studies that show an association between homes with humidity problems and asthma symptoms in children lead some healthy-house advocates to promote high ventilation rates. That might make sense in Sweden in January, when outdoor air is dry. But the same advice can’t be applied to Houston in July.
According to , “A higher indoor humidity, which, in turn, can lead to more indoor dust mites (an important allergen source) and to a greater risk of indoor mold growth, is another potential consequence of increased ventilation rate. Indoor humidity will increase with ventilation rate only when the outdoor air is more humid than the indoor air, e.g., during hot humid weather, and when the building mechanical systems also do not dehumidify sufficiently to counteract the effects of increased moisture entry. When outdoor air is less humid than indoor air, e.g., during cool winter weather, more ventilation decreases the indoor humidity.”
Overventilation carries an energy penalty
Healthy house advocates often sing the praises of high ventilation rates. But it’s important to remember that overventilation has several downsides.
The penalties associated with overventilation are summed up succinctly by William J. Fisk, Douglas Black, and Gregory Brunner, in an article titled The authors wrote, “Providing more ventilation increases building energy consumption, increases the related emissions of carbon dioxide, and contributes to climate change. Modeling of the U.S. commercial building stock indicates that 6.5% of all end-use energy (3.2% in offices) is for heating and cooling of mechanically-supplied outdoor air ventilation. … One can estimate that an additional 3% of total end-use energy is used to heat and cool infiltration air, thus, an estimated 9.5% of end use energy is required for ventilation.”
What do you do when outdoor air is polluted?
It’s also important to remember that outdoor air isn’t always clean. In some locations, outdoor air can be contaminated by vehicle exhaust. Outdoor air can be high in ozone and high in particulates. notes, “Indoor concentrations of some outdoor air pollutants can increase with ventilation rate. Increases in indoor ozone concentrations may be most significant. Higher outdoor air ozone concentrations are associated with adverse respiratory and irritation effects and several other health effects. Outdoor air polluted with ozone is normally the major source of indoor ozone. Because ozone is removed from indoor air through chemical reactions with indoor pollutants and materials, indoor ozone concentrations tend to be substantially lower than outdoor air ozone concentrations; i.e., buildings tend to shield us from outdoor ozone. However, as ventilation rates increase, indoor ozone concentrations become closer to outdoor concentrations. Thus, increasing the ventilation rates will increase our exposures to ozone.”
The same web site notes, “Increases in ventilation rates will also generally increase indoor concentrations of, and exposures to, outdoor air respirable particles, while simultaneously reducing our exposures to indoor-generated particles. Higher outdoor particle concentrations are associated with a broad range of adverse health effects. If the incoming outdoor air is filtered to remove most particles, the influence of ventilation rate on indoor particle concentrations can be small. Ventilation rates, if stable over time, will not generally affect time-average indoor concentrations of non-reactive gaseous outdoor air pollutants such as carbon monoxide, but higher ventilation rates can increase peak indoor concentrations.”
The dangers associated with outdoor ozone and respirable particles only occur in some locations. The web site notes, “It is clear that more ventilation can increase our exposures to some pollutants, particularly where and when the outdoor air is highly polluted or warm and humid. At the same time, the increases in ventilation rate will diminish our exposures to a variety of indoor-generated air pollutants. On balance, the scientific literature points to improvements in health and performance with increased ventilation rate; however, at polluted locations where it may not be possible or practical to adequately remove pollutants from incoming ventilation air, it is possible that some moderate intermediate ventilation rate is better for health than higher ventilation rates.”
, increasing a home’s ventilation rate is an unsophisticated way to address indoor pollution problems. “So what is the typical recommendation from one of these reports?” Lstiburek writes. “Increase the ventilation rate. That will reduce the concentration. Aaragghh. Dilution is not the only solution to indoor pollution. We can’t ignore the effect of ventilation rates on energy and part load humidity. We can’t just turn up the crank and ventilate like crazy. Whatever happened to source control?”
Can we determine the best ventilation rate by testing indoor air?
The idea behind mechanical ventilation is to improve the quality of indoor air. If a house doesn’t have enough ventilation, then indoor air should be bad — right? And if the ventilation system is working well, then indoor air should (presumably) be good. And there should be a simple way to test indoor air to see if it’s good or bad — right?
Unfortunately, the facts don’t conform to this simplistic view.
For one thing, the main reason that indoor air is “bad” has nothing to do with a home’s ventilation rate. It has to do with occupant behavior.
For another thing, it’s hard to test indoor air. , “I don’t have a problem with measuring CO2 — it is sometimes, not always, a pretty good surrogate for ventilation rates. … I also don’t have a problem with measuring temperature and relative humidity — I can tell a lot from measuring temperature and relative humidity. … At this point I typically stop with the testing. Why? Almost everything else is pretty much a waste of time. The most popular waste of time are tests for volatile organic compounds (VOCs).”
If you follow the link in the above paragraph, you can find out why Lstiburek believes that testing for VOCs is a waste of time. Here’s the gist of Lstiburek’s argument (with a few of my own opinions thrown in): if you want high-quality indoor air, don’t pollute the air in the first place. Don’t smoke tobacco. Don’t light candles. Don’t use plug-in air fresheners. Don’t clean indoor surfaces with solvents. Don’t spray insecticides indoors.
And if you know that there is a source of pollution in your home, install an exhaust fan close to the pollution source and use it appropriately. Taking these common-sense steps make a lot more sense than increasing your home’s ventilation rate.
Indoor air pollutants of concern
Fortunately, researchers have studied the question of which indoor air pollutants are most concerning. According to “Healthy Efficient Homes: Research to Support a Health-Based Residential Ventilation Standard,” by Brett C. Singer, Jennifer M. Logue, and Max H. Sherman, the four pollutants that are most concerning in “non-radon homes” are acrolein (a byproduct of burning tobacco, firewood, candles, and foods), PM 2.5 (fine particulate matter), NO2 (nitrogen dioxide, which is formed when natural gas is burned), and formaldehyde. The researchers conclude, “In-home air pollutant health risk dominated by PM 2.5. … Currently quantifiable chronic air pollutant health risk in non-smoking, non-radon homes driven mainly by PM 2.5.”
What’s the takeaway? If you have a supply ventilation system, the system should include a filter; and if you have a gas range, use your range hood fan. The researchers wrote, “In homes that cook with gas burners, ~50% exceed 1-h NO2, ~5% exceed CO standards.”
The bottom line
After researching existing data on the correlation between residential ventilation rates and occupant health, I have concluded that healthy-house advocates often overstate the advantages of high ventilation rates. At this point, we just don’t have enough data to make health claims for high ventilation rates. We need more research on this issue.
That doesn’t mean that we can ignore the need for mechanical ventilation in homes. I think it’s wise for builders to install equipment that allows occupants to ventilate their homes at the rate recommended by ASHRAE 62.2 (7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area). However, that doesn’t mean that every home in the U.S. needs to be ventilated at that rate.
If you care about indoor air quality, the most important thing you can do is to limit the release of pollutants — especially tobacco smoke — inside your home.
The next most important thing you can do is to employ exhaust fans for source control: for example, to remove humid air from bathrooms, to remove combustion gases from the area above your kitchen range, and to remove any noxious fumes given off by indoor hobbies.
Once these steps are taken, you’ll probably still want to operate ventilation fans — either at the ASHRAE 62.2 rate, if your lifestyle generates lots of odors or particulates, or at a rate below the ASHRAE 62.2 rate, if your lifestyle is on the non-polluting end of the spectrum.
However, these fans probably don’t need to run when no one is home, and you’ll probably want to limit use of ventilation fans during hot, humid weather.
Martin Holladay’s previous blog: “Pearls of Wisdom From Recent Conferences.”