Editorial

What we don�t know

Nasim Mullen, who is one of my PhD students, spentlast summer in China working in cooperation withProfessor Yinping Zhang�s group at Tsinghua Univer-sity. Her project was to investigate ultrafine particleconcentrations inside high-rise apartments in Beijing.One of the interesting stories Nasim related from thatexperience was this: at most of sites she studied, theresidents regularly kept their windows open for naturalventilation. But at one apartment, the dwellersexpressed concern that the outdoor air was toopolluted. In response, they kept their windows anddoors tightly shut and used a recirculating, high-efficiency air filter to reduce indoor particle levels.Which of these practices is better for indoor environ-mental quality and health? The monitoring datashowed that the closed apartment with air filtrationhad markedly lower particle number concentrations.But that is only one small aspect of indoor environ-mental quality.This anecdote is symbolic of a broader divide in the air

quality and environmental health communities. Let mepresent it here in caricature. The indoor environmentcommunity tends to think that important air pollutionproblems originate from indoor sources. From thisperspective, more ventilation is better. We know thatindoor air quality (IAQ) problems occur when indoorsource emissions are excessive regardless of the ventila-tion rate. And we also know that IAQ problems resulteven when indoor emissions are in a normal range ifventilation is inadequate. However, the desire for moreventilation is constrained by concern about the energythat is required, its costs and especially its environmentalimpacts.By contrast, the larger outdoor air research and

professional community tends to think that people areexposed to outdoor pollution at outdoor levels. Fromtheir perspective, the fact that people are almost alwaysindoors is irrelevant for characterizing air pollutionexposure. In this worldview, buildings do not alterexposure to outdoor pollution. Indoor sources, to theextent that they exist, have not been proven to besignificantly harmful to human health when comparedwith outdoor air pollution.In their extremes, both of these perspectives are

wrong and potentially harmful. A quote from the 19th

century, US author, Mark Twain, applies: �It ain�t whatyou don�t know that gets you into trouble. It�s whatyou know for sure that just ain�t so.�What is the truth? Most people live in urban areas.

Urban outdoor air is commonly polluted to degreesthat pose health risks. (So too is rural air in manylocales.) But indoor sources also emit pollutants intoindoor spaces that adversely affect human health. Highventilation rates, absent effective pollutant removal,drive indoor air to become more like outdoor air, forbetter and for worse. Conversely, with low ventilationrates, the indoor environment becomes progressivelydecoupled from the outdoor environment, again forbetter and for worse. In general, source control is thefirst best option for improving both indoor andoutdoor air quality. Indoors, ventilation serves as animportant complementary measure to source controlfor establishing and maintaining good environmentalquality. However, when the outdoor air is polluted, oneencounters that difficult position where too littleventilation may be insufficient to remove pollutantsfrom indoor sources, but too much, without effectivepollutant treatment, impairs the effectiveness of theindoor environment to provide protection against(some) outdoor pollution. What is the right ventilationrate for buildings in a case like this, which is all toocommon? Honestly, I do not think we know.Even on the topics like this one – the relationships

between building ventilation, indoor air quality andhealth – where we have a foundation of theoretical,empirical and practical knowledge, there is much thatwe do not know. There are many other topics in therealm of indoor environment and health where we notonly lack knowledge, we even seem to lack awareness ofthe potential importance of the issue. In the followingparagraphs, I comment on two examples of the latter.A dominant cause of mortality in modern society is

heart disease. In the United States, �diseases of theheart� has been the leading cause of death for at leastthe past half century, accounting for 25% of all deathsin 2007. In addition to the high prevalence, a strikingfeature of heart-disease statistics has been the strongand persistent decline in the age-adjusted mortalityrate. For example, US heart-disease mortality de-creased from 266 deaths per year per 100,000 people in

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1999 to 191 deaths per year per 100,000 people in 2007(http://www.cdc.gov/nchs/deaths.htm). That is a de-cline of almost 30% in the most prominent cause ofdeath in the United States in just under a decade!Parallel declines have occurred in many countries thathave a high �human development index.� This evidenceof strong improvement in an important health statisticis at least somewhat surprising, given other indicators,such as the coincident rise in obesity and the attendanthealth problems from that epidemic. Meanwhile, arecent paper referenced the �recent, dramatic increasesin coronary heart disease mortality in China,� ascribingthe increase to �adverse changes in major cardiovascu-lar risk factors� (J Cheng et al., BMC Public Health2009, 9:30 doi:10.1186/1471-2458-9-30). That studycomments on diet and smoking as major factors inthe differential temporal patterns. Not mentioned hereor elsewhere is the potential role of shifting exposure toenvironmental tobacco smoke (ETS) as a potentialcontributor to these patterns. ETS exposure – verymuch an issue of indoor environment and health – hasbeen implicated as an important risk factor in acutemyocardial infarction (JM Lightwood and SA Glantz,Circulation 2009; 120: 1373–1379). Cultural normsabout exposure to ETS have changed dramaticallyover the past decades, most notably in the UnitedStates and in Europe. Just to illustrate, consider thesesmall personal notes. Two decades ago, as I wasfinishing graduate school, one of my future colleaguessmoked a pipe in his office during my interview withhim. When I traveled to Indoor Air �93 in Helsinki, Iwas placed in the row immediately in front of thesmoking section on the flight from New York. Now, Ican go a week without encountering even a whiff ofETS. Might declines in ETS exposure be a significantcontributor to the decreasing death rates from heartdisease in the United States and elsewhere?I have also been musing recently about the possible

roles of carbon dioxide as an indoor air pollutant.Carbon dioxide is an acid gas, and at high levels,�inhalation of CO2 can produce physiological effects onthe central nervous, respiratory and the cardiovas-cular systems� (http://www.osha.gov/dts/sltc/methods/inorganic/id172/id172.html). In the indoor air commu-nity, CO2 is mainly used as one gauge of the adequacyof ventilation. For example, in the most recentASHRAE ventilation standard, it is reported that�maintaining a steady-state CO2 concentration in a

space no greater than about 700 ppm above outdoorair levels will indicate that a substantial majority ofvisitors entering a space will be satisfied with respect tohuman bioeffluents� (ANSI/ASHRAE Standard 62.1-2010). However, recognizing its potential for frankadverse health effects, the US Occupational Safety andHealth Administration maintains an occupationalstandard for CO2, with a �transitional limit� of 5000ppm for the eight-hour time-weighted average (http://www.cdc.gov/Niosh/pel88/124-38.html). As a matterof sound public policy, occupational standards forpollutants are typically set at much higher levels thanwould be appropriate for the general public. Mightthere be some adverse health consequences associatedwith exposures to CO2 at the levels at which they occurindoors? A recent study by Beko et al. (Buildingand Environment 10.1016/j.buildenv.2010.04.014) usedmeasurements of CO2 in the bedrooms of 500 Danishchildren to characterize ventilation conditions duringsleep. Surprisingly, they found that 6% of the roomshad levels that exceeded 3000 ppm (20-min runningmean) at some time during the night. Does exposure toelevated CO2 levels during sleep matter for health? I donot think we know. But consider these points.Obstructive sleep apnea, which reflects one type ofsleep disturbance that would influence the balance ofmetabolic gases, has been implicated as an importantrisk factor in cardiovascular morbidity and mortality(P Lavie and L Lavie, Current Pharmaceutical Design2008, 14, 3466–3473). Furthermore, the existence of acircadian variation has been demonstrated in severalcardiovascular outcomes including myocardial infarc-tion, �sudden cardiac death, symptomatic and asymp-tomatic myocardial ischemia, and stroke, all of whichhave been shown to occur more frequently during themorning than at other times of day� (SN Willich et al.,Circulation 1989; 80: 853–858). Is it possible thatdegraded indoor air quality in one�s bedroom duringsleep plays a role in adverse health effects such as these?The pages of this journal are intended to chronicle

the new knowledge that we create concerning indoorenvironment and health. As a community, to use thisresource efficiently and wisely, we need to understandclearly what is already known. And some times, thebest way to understand what we know is to focus ourattention on what we don�t know.

William Nazaroff

Editorial

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