Max Sherman is a senior scientist in the Residential Building Group at Lawrence Berkeley National Laboratory (Berkeley Lab). As part of the group's Healthy Efficient Homes project, he and other project researchers examine the interrelationships among HVAC and indoor pollutants. In recent years, the group has advanced knowledge in this area by not only identifying the most prevalent indoor pollutants, but also by developing methods to better quantify health effects from those substances. Max draws on that knowledge when considering building envelope tightness and ventilation standards for energy-efficient homes to improve indoor air quality and occupant health. He serves on the ASHRAE Standing Standard Project Committee 62.2, which provides input for revisions to the ASHRAE 62.2 residential ventilation standard.
Some years ago, the Residential Building Group identified the nine highest-priority pollutants in U.S. homes. However, the health impacts from exceeding each pollutant's standard differ from one another, correct? How did you resolve that?
The first task was to identify the most important non-biological airborne contaminants of concern, which we did in 2009. Then it was important to determine what kind of impact each of these contaminants have on people. A lot of work had already been done in this area, although it had not been applied to look at the impacts of indoor air in homes. We used existing knowledge of the health impacts of exposure to pollutants to determine impact of air in homes on occupants. Beyond that we needed to find a metric that would allow us to compare the impacts of these different substances, because the health outcomes of exposure were not all equal. Not all standards are created equal. The levels deemed as "safe" do not correspond to the same chance of disease occurring and the health outcomes, and resulting impact on quality of life, for exposure to different pollutants are very different.
Your group chose "disability adjusted life years," or DALYs, as the measure of how indoor chemicals can affect human health. Could you explain this concept and how it applies to this work?
DALYs are a metric for quantifying the harm from disease, illness, or injury. In our case we used them to quantify the health burden of illness resulting from exposure to a pollutant. They're based on an optimal life span based on good heath and the difference from that condition as the result factors that affect one's health. So, for example, if a substance in your home causes a person to die two years before the time they otherwise would, that is measured as 2 DALYs. The higher the number, the more serious it is. But it doesn't only address mortality; DALYs can be used to measure reduced quality of life as the result of an illness as well. So if a person experiences a five-year illness that reduces their quality of life to four-fifths of a what it would be in a healthy year, that is considered one DALY lost. By using the DALY metric, we're able to see which of the priority indoor air pollutants are causing the most health problems and focus our attention on those first, to achieve the greatest overall impact.
From this work, you've concluded that fine particulates—PM2.5—cause the most health damage. Is that because they are more widespread than other pollutants, or are there other reasons? What health problems does PM2.5 cause?
Particle emissions are clearly at the top of the list. Both of the reasons you mentioned contribute to this—PM2.5 is more widespread than the other pollutants, and it also creates more health problems. Our most recent study found that PM2.5 contributed significantly to DALYs lost due to stroke, chronic bronchitis, and premature death. Particulates exist in both inside air and outside, air, although not necessarily from the same sources. Indoor PM2.5 stems mostly from indoor combustion; cooking or burning candles, that kind of thing. Outdoors, it comes from other sources, such as diesel vehicles or power plants or fires. Because it's so ubiquitous, it's difficult to set a standard that can be reasonably met, but standards-setting organizations do their best, based on the information available.
What additional research needs to be done to better understand PM2.5, to protect public health?
Data on outdoor PM2.5 is plentiful, but those on indoor PM2.5 are less so. Until recently, most researchers didn't think it mattered. The question now is whether or not the indoor PM2.5 may react differently than that outdoors. It may, or it may not—future studies will tell us that. Particles larger than 2.5 microns do not go into the lung, so they are not as big a health problem, but particles smaller than that can get into the lung and cause serious problems. Particles smaller than 0.1 microns (which are referred to as ultrafine particles) can even go through the lung wall and into a person's body. Those may even have different biological mechanisms—we just don't know yet. So it would be helpful to have better health impact data on those very small particulates. Berkeley Lab doesn't conduct that kind of research, but we'll certainly take that kind of information into account when it's available.
From a ventilation perspective, we can filter particulates larger than 0.3 microns fairly effectively and inexpensively, but below that size it's more difficult and more expensive to filter them out. So when we consider indoor air quality, it's important to know: do most of the harmful health effects come from the very small particulates or from that range of 0.3 microns to 2.5 microns? Where is the most damage coming from? If it's the former, then we need to develop more effective and cost-effective means of reducing those extremely fine particulates. But if it's the latter, the technology is already available, and we just need to find the best ways to apply it.
Next on the list of the worst pollutants are acrolein and formaldehyde. Why are these present in people's homes, and what kinds of health effects result from their presence?
In our study, formaldehyde and acrolein were estimated to have the largest number of DALYs, so they're a major problem. Inside the home, acrolein is most often the result of incomplete combustion, from cooking foods (especially oils), burning biomass (such as wood), and secondhand smoke. It irritates the lungs; in fact, it was used as a chemical weapon in World War I, and is the only gas used as a weapon that has not been banned, because it's a natural combustion by-product and so is difficult to isolate and regulate in that way.
Formaldehyde is emitted by materials used in home construction, such as particle board and paneling. It can also come from foam insulation and tobacco smoke. It is a lung irritant and can trigger asthma attacks. It also causes cancer in other animals, and may cause cancer in humans.
These studies also identified three contaminants—secondhand smoke, ozone, and radon—that were not on the original list of priority indoor contaminants but cause site-specific health problems in homes. In the homes where they are a problem, where do these stand in comparison to the priority pollutants?
Although our studies showed that in the majority of U.S. residences PM2.5, acrolein, and formaldehyde dominated the health impacts attributable to non-biological air pollutants, second-hand smoke and radon contribute significantly to DALYs in the homes where they're a concern. In those instances, they are a priority, and are more important to deal with than, say, formaldehyde.
Of course, ventilation is a primary method for improving indoor air quality because it can reduce or dilute environmental pollutants. However, ventilation standards such as ASHRAE 62.2 don't consider specific removal of the priority pollutants. How would we benefit from revising the ASHRAE 62.2 standards to incorporate a health-based indoor air quality standard, and how might that work?
ASHRAE 62.2 provides guidance for a ventilation rate based on a particular type of building and other factors, but a health metric isn't part of that calculation. Incorporating a health metric such as DALYs into the standard would allow designers and builders to consider the building materials used and other factors when determining ventilation rates. If it were clear that those factors showed lower indoor emissions, then a lower ventilation rate could be used; if not, you'd use a higher rate.
You could also consider the trade-off between contaminants to find a balance of emissions that is acceptable. That is, if formaldehyde emissions were predicted to be low because of the materials used, a higher level of another contaminant might be acceptable. If post-construction levels were higher than estimated, or if subsequent occupants used furnishings that emitted higher levels of contaminants, then the standard could allow for additional air filtering.
The goal is to find a health-based ventilation standard that is flexible enough to address site-specific differences while ensuring healthy indoor air quality and optimal energy efficiency. In a mild climate, it might be appropriate to use a lot of air for ventilation, because it doesn't need much conditioning. But in Alaska, you want to use less ventilation, because it's going to be energy intensive and expensive to condition it, and rely on other strategies for improving indoor air quality.
I'm part of the committee that revises ASHRAE standard 62.2. The most recent standard is being released in the summer of 2013, and revisions are made on a three-year cycle, so we'll incorporate the health-based approach in the next version. It may even take a couple of revisions cycles.
What is the most important conclusion of this work?
There are two I think. First, that DALYs from PM2.5, acrolein, and formaldehyde combined are much greater than those attributable to all the rest of the remaining 67 indoor air pollutants that we've identified combined. That certainly makes a strong case for focusing attention on those contaminants when addressing indoor air quality.
The second point is that, although there's still much to be learned about indoor air quality, we've made significant progress in recent years, and we need to act on that wealth of new information. Whether it's revising standards or evaluating the best building materials, this new health-based information gives us a much better picture of what constitutes healthy indoor air, and we can develop more energy-efficient, healthier homes by targeting the biggest problems.
Logue, J.M., T.E. McKone, M.H. Sherman, and B.C. Singer. 2010. "Hazard Assessment of Chemical Air Contaminants Measured in Residences." Indoor Air 21:92 109.
Logue, J.M., P.N. Price, M.H. Sherman, and B.C. Singer. 2011. A Method to Estimate the Chronic Health Impact of Air Pollutants in U.S. Residences. Environmental Health Perspectives. LBNL-5267E.