There is an unvented combustion appliance that people use every day that is often ignored
Many of the trade-offs in the home performance industry have to do with health, cost, and energy consumption. Even though we are passionate about helping clients reduce energy consumption and cost, few of us would ever recommend that they trade health for either.
Home performance pros understand the need for adequate ventilation in homes as building envelopes have become tighter. Whole-home mechanical ventilation removes pollutants generated inside tight homes; its installation has become standard best practice in much of the United States (though it’s still not as common as it should be).
In addition to the need for general, whole-home mechanical ventilation, the home performance industry has long—and justifiably—been concerned about the pollutants being generated by combustion appliances in the home, and it understands the importance of ensuring that those pollutants are exhausted to the outside to reduce occupants’ exposure.
And yet, although we recognize both the need for mechanical ventilation and the need to exhaust combustion pollutants, there remains an unvented combustion appliance that many of our clients use every day but to which we rarely give a second thought. I’m referring, of course, to a gas-powered cooking range.
Cooking Pollutants Are a Problem
A 2011 report from the Lawrence Berkeley National Laboratory (LBNL), where I work as a building science researcher, evaluated common indoor pollutants and found that reducing exposure to cooking pollutants represented one of the biggest opportunities for improving air quality in homes. (For citations for the 2011 report and other research referenced in this article, see the link to the full research report on which this article is based, under “learn more.”) The 2011 report found that it’s not just the pollutants from combustion that are problematic, but also the pollutants generated from cooking itself. This means that electric-resistance cooktops are not off the hook. Induction cooktops may produce fewer pollutants because they operate at lower temperatures, but they too need to be vented. It’s not just cooktops that generate pollutants, but ovens too (see Figure 1). (BPI test protocols include oven CO measurements—for more on this, see the link to BPI test procedures under “learn more.”)
Pollutants Generated by Cooking
The pollutants generated by cooking vary by cooktop but can include NO2, CO, formaldehyde, fine and ultrafine particles, acrolein, and polycyclic aromatic hydrocarbons. Cooking also generates water vapor, which is not a pollutant in itself but can contribute to moisture-related indoor air quality (IAQ) problems.
The United States has no consensus IAQ standards, so we often refer to EPA’s National Ambient Air Quality Standards (NAAQS) for outdoors to gauge the level of concern for indoor air pollutants. Measured short-term concentrations of NO2 regularly generated from cooking events exceed the NAAQS maximum 1-hour exposure level by a factor of 10. Measured short-term concentrations of fine particles (PM2.5) from common cooking events exceed the NAAQS maximum 24-hour exposure level by a factor of nearly 100. (See the link under “learn more” for more on the EPA standards.)
In other words, if the concentration of these pollutants present indoors in our homes—where we spend a lot of time—were present outdoors, they would be in violation of the Clean Air Act.
Health Effects of Exposure to Cooking Pollutants
Short-term exposure to NO2 is associated with airway inflammation in healthy people and increased respiratory symptoms in people with asthma. Exposure to fine particles is associated with decreased lung function and increased respiratory symptoms such as airway irritation, coughing, or difficulty breathing. In especially susceptible populations—such as people with existing heart or lung diseases, children, and the elderly—exposure to fine particles is associated with more-serious health problems, including irregular heartbeat and even heart attacks.
What Can We Do to Reduce Exposure to Cooking Pollutants?
The most straightforward way to effectively remove cooking pollutants from the living space is to have a vented range hood and to operate it each time you cook. This seems simple. But the more we’ve looked at this issue, the more complex this seemingly simple problem has revealed itself to be. In practice, there are a lot of impediments to making effective kitchen ventilation a reality in all homes. We do think there are solutions, but they are multifaceted and will likely take some time to implement.
For now, to address the problem of exposure to cooking pollutants, the first step is to acknowledge that the problem exists. Our bodies’ senses fail us in this respect. We have evolved to recognize and respond to immediate acute threats to our health and safety. We are not as good at recognizing the risks of exposure to chronic, low-level pollutants that do not represent an immediate threat but in the long term may shorten, or decrease the quality of, our lives. We rely on the scientific method to enable us to overcome this immediacy bias and recognize long-term risks that our physical senses fail to detect. Empirical observation has enabled us to identify the chronic risks of tobacco smoke, radon, atmospheric ozone, greenhouse gas emissions, lead, and asbestos, among countless other examples, none of which very real risks are apparent to our senses alone. Now this same approach has helped us understand the risks of long-term exposure to cooking pollutants.
One of the central roles of the home performance professional is to enable building occupants to reduce their exposure to chronic pollutants. For cooking pollutants, this means ensuring that an effective vented range hood is installed, and it means educating occupants on the importance of using it. Having myself inexplicably broached this topic with friends unwise enough to invite me into their living spaces, I understand how poorly it is likely to be received. In time, we can only hope that building codes and standards, and even range hood marketing and product ratings, will help to reinforce this message, and lighten our burden of educating the consumer.
What Makes for Effective Kitchen Ventilation?
A collaboration of research institutions is currently in the process of attempting to determine what exactly makes for effective kitchen ventilation, and how to measure that efficacy. While we cannot yet answer this question in detail, we can now list the three main criteria. To be effective, kitchen ventilation should consist of 1) a range hood that 2) is vented to the outside and that 3) is turned on every time the cooktop or oven is used.
It is important that the fan be a range hood (rather than a general exhaust fan or downdraft fan, for example), because the hot plume of pollutants that is generated from cooking tends to rise and spread out. A vented range hood captures the plume as it rises, sucks it out of the house, and vents it outside. Other kinds of exhaust fan can do this too, but they don’t do it as well or as efficiently.
It is important that the range hood be vented to the outside because that gets the pollutants out of the living space. The recirculating unvented range hoods that are ubiquitous in new homes do little if anything to reduce occupants’ exposure to cooking pollutants.
A range hood has to be turned on to be effective. Whether or not the range hood is turned on at every cooking event is largely determined by how loud the fan is, by the occupant’s perception of the need to use the fan, and by habit. The noise generated by the fan is a factor of the range hood’s design, the speed at which the fan is set, and the ductwork that the fan is connected to. Occupants do not generally understand the relationship between the cooking activity being performed—frying, boiling, sautéing, and so on—and the risk of exposure to kitchen pollutants. Habits, good and bad, take time to develop and are difficult to change. Some currently available high-end range hoods operate automatically as needed. In time, base-level models may do so too.
If you have a vented range hood that you use every time you cook, you have substantially decreased your exposure to cooking pollutants. If we could make this the case in every U.S. home, it would represent a substantial improvement in residential IAQ and a substantial reduction in related diseases. A quiet, vented range hood should be the minimum performance requirement in new homes and retrofits. Figure 2 gives a simple flowchart you can use to determine if yours or a client’s kitchen ventilation is providing an environment that is free of unhealthy concentrations of cooking pollutants.
But there are ways to increase the efficacy of kitchen ventilation beyond this baseline level of performance. These strategies include 1) cooking primarily on back burners, 2) ensuring that your range hood extends out to the front burners, 3) using a range hood with a deep sump (that is, one shaped like an inverted bowl, not a flat plane), and 4) using a range hood whose fan moves at least 200 CFM (while still being quiet enough to allow for conversation). Figure 3 shows some ways to increase range hood efficacy.
Cooking on the back burners and using a range hood that extends out to the front burners both help to ensure that the cooking plume is exposed to the suction of the fan as it rises, increasing the likelihood that the pollutants will be captured and exhausted. The deep sump helps to physically contain the plume. Increasing the airflow extends the reach of the capture hood into the room and increases the hood’s capacity to exhaust pollutants. A recent study found that range hoods moving 250 CFM can effectively capture the pollutants generated by two burners (each ≤16,000 Btu/hr) on a standard residential cooktop.
Energy Consumed by Operating Vented Range Hoods
Operating a range hood will increase a home’s energy consumption, but by how much? Another recent LBNL study sought to estimate the answer to this question. The authors found that most of the energy increase is associated with conditioning the makeup air and replacing the heat that the cooking range was providing during the heating seasons (net the increased cooling load it was generating in cooling seasons). Less than half of the total energy increase is associated with the fan power of the range hood itself.
The overall increase varied according to climate zone and the current range hood use rate for each region, but in the aggregate, it was estimated that if every U.S. home regularly operated a vented range hood, total residential site energy use would increase by roughly 1%. The average increase to each home’s energy costs was estimated to be around $15 per year.
This represents our best current understanding of the physical characteristics of an effective kitchen ventilation system. But even though we now know what effective kitchen ventilation looks like, there are many barriers to getting it installed and operating in homes. The most important of these barriers are discussed below.
Where Does the Demand for Effective Range Hoods Come From?
Are homeowners going to take it upon themselves to become educated about the health effects of chronic exposure to cooking pollutants and come out en masse demanding that home builders and range hood manufacturers supply and install affordable, effective range hoods? This seems unlikely. Maybe a few will, but the vast majority of the population is probably more concerned with other matters. To get effective kitchen ventilation disseminated into U.S. homes, initial demand will probably need to be generated by a combination of government and industry efforts. Currently, few states have mandatory codes or standards relating to kitchen ventilation. The most widely used standard that has requirements pertaining to kitchen ventilation is ASHRAE 62.2. This standard has had a significant impact on improving IAQ in jurisdictions in which it is used, and it is much better than no ventilation requirements at all; but its kitchen ventilation requirements could be improved both to ensure more effective ventilation and to make enforcement by local code officials more straightforward and more likely.
Most States Lack a Kitchen Ventilation Requirement
The majority of states use the International Residential Code (IRC) or the International Mechanical Code (IMC) as the basis for their state residential building codes. But neither the IRC nor the IMC requires the installation of kitchen ventilation.
Lack of Code Enforcement
Even though ASHRAE 62.2 is required in California by state building codes, it is likely that very few new homes actually meet the standard’s kitchen ventilation requirements. ASHRAE 62.2-[2010 and 2013] requires that intermittent kitchen ventilation fans be rated by the Home Ventilating Institute (HVI) for sound (3 sones or less) at 100 CFM. As of this writing, no microwave range hood models meet these HVI rating levels, so no microwave range hood models are ASHRAE 62.2 compliant. (See the link under “learn more” to learn about HVI ratings.)
There’s more. To use ASHRAE 62.2’s prescriptive duct-sizing pathway (as opposed to manually measuring range hood airflow—not an easy or quick task), the range hood must be HVI rated for 100 CFM62.5 (0.25 IWC). As of this writing, of the 4,000 range hoods listed in the HVI directory, 5 have an airflow rating at 62.5 Pa of pressure.
Even though ASHRAE 62.2 is required in new California homes, it’s apparent that few new homes actually meet the standard’s kitchen ventilation requirements. I’d love to be proven wrong on this point. But in any case, this example reminds us that codes and standards are only effective when they are enforced, so we should make their enforcement as straightforward and transparent as possible.
As more states and programs come to require ASHRAE 62.2, we need to do more nationally both to measure compliance rates and to lower barriers to compliance, to ensure that the standard’s adoption is improving the actual performance of homes.
Lack of Information
Currently consumers, home builders, or architects who are looking for an effective range hood find it hard to get the information they need to determine efficacy. The only performance information that is widely available for range hoods is the manufacturer’s stated airflow and sound level, and perhaps the HVI-rated airflow and sound level. These performance metrics are of little use. The manufacturer’s stated performance levels bear little resemblance to actual installed performance. The HVI-rated numbers better reflect installed performance, but airflow rate is only one of the factors—albeit an important one—that determine the efficacy of a range hood.
What is needed is a performance test that directly measures a range hood’s ability to capture and remove cooking pollutants. ASTM is currently developing the method for such a test. In time, we hope that this capture efficiency rating will be incorporated into appliance standards, and will be listed on the label for each range hood model.
If more states begin requiring kitchen ventilation, and if standards are improved and building codes more routinely enforced, effective kitchen ventilation could become routine in new homes. But there are confounding issues that could prevent this from happening in some new homes, and especially in existing homes.
As envelope airtightness requirements for new homes become more stringent, vented range hoods will be more likely to cause significant depressurization. We need to anticipate and address this issue, first by eliminating natural-draft combustion appliances from the living space, and second, by providing a dedicated makeup air pressure relief damper to replace the air being exhausted by the range hood. This makeup air may need to be tempered in some climates.
In supertight passive and near-passive houses, the trend has been to use recirculating range hoods or to rely on the HRV/ERV return in the kitchen to exhaust cooking pollutants. Based on our observations, we believe that these strategies are inadequate. A vented range hood with a high-quality pressure relief damper that will not compromise envelope airtightness is a better solution.
Many existing homes lack adequate ductwork to allow for the installation of kitchen ventilation. In these cases, improved, low-resistance ductwork should be installed with a vented range hood.
The most problematic existing homes have no ductwork at all and lack the physical space to add ductwork. This condition is especially common in high-rise multifamily apartment and condominium buildings, where the unit’s kitchen may be far from any exterior walls, and where vertical venting is impeded. In these particular cases, installing a vented range hood may be effectively prohibited by cost, or explicitly prohibited by the rules of the homeowners’ association.
The full research report on which this article is based, Stratton, J. Chris, and Brett C. Singer. Addressing Kitchen Contaminants for Healthy, Low-Energy Homes, LBNL-6547E. Berkeley, CA: LBNL, 2014, is available for download at Berkeley Lab’s Environmental Energy Technologies Division website, http://eetd.lbl.gov/publications.
Download BPI's “CO Safety Test Procedures for Vented Appliances.”
Get more on EPA’s NAAQS.
Download more information about the study on range hood flow rates.
To learn about HVI ratings.
Until these buildings undergo substantial retrofit or are decommissioned altogether, there are some stopgap filtration (rather than ventilation) strategies that occupants can use to reduce their exposure to cooking pollutants. While these strategies fall short of the protection provided by a vented range hood, they are better than no protection at all. These strategies include installing stand-alone air filtration devices or a robust unvented range hood that mechanically removes grease and captures pollutants using both activated carbon and a pleated filter. Regular maintenance and filter replacement is required to ensure that these strategies are as effective as possible.
Kitchen ventilation is emerging as one of our best opportunities to reduce exposure to a hazardous pollutant that is generated in every home. It will take time to further develop test methods, standards, and codes that properly characterize and address the removal of cooking pollutants, but the process is under way.
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