From Good Intentions to Proven Interventions: Effectiveness of Actions to Reduce the Health Impacts of Air Pollution

Luisa V. Giles; Prabjit Barn; Nino Künzli; Isabelle Romieu; Murray A. Mittleman; Stephan van Eeden; Ryan Allen; Chris Carlsten; Dave Stieb; Curtis Noonan; Audrey Smargiassi; Joel D. Kaufman; Shakoor Hajat; Tom Kosatsky; Michael Brauer

Disclosures

Environ Health Perspect. 2011;119(1):29-36. 

In This Article

Interventions Directed to Individuals

In addition to the air quality management strategies focused on emissions reductions and local initiatives to control sources and separate them from residential locations, schools, and health care facilities, the workshop highlighted the value of lowering baseline health risks to reduce pollution-related health impacts. Specifically, the implementation of established primary, secondary, and tertiary interventions (e.g., controlling hypertension, lowering lipids, reducing obesity, promoting physical activity and smoking cessation) for diseases affected by air pollution exposure will serve to reduce the overall burden of disease associated with air pollution exposure. For example, sedentary individuals and those with a diet deficient in antioxidants or with a high salt diet may have an increased risk of developing cardiovascular disease (Marchioli 2003; Qin et al. 2009; Warburton et al. 2006) and may therefore also be more susceptible to the effects of air pollution. Through diet modification, exercise, and possibly via antioxidant supplementation, individuals can potentially reduce their personal susceptibility. Several of these approaches are discussed in more detail below.

Exercise

In most developed countries, both air pollution and physical inactivity pose significant health risks to urban populations. The benefits of regular exercise on cardiovascular disease incidence and progression are unequivocal (Warburton et al. 2006). Active transportation (e.g., walking or cycling to work) can increase physical activity and reduce the burden of cardiopulmonary disease; however, the health benefits of active transport may be partially compromised if location and potential air pollution exposure are not also considered. Exercise leads to increased PM inhaled dose (Panis et al. 2010) and, under some circumstances, to increased PM deposition (Daigle et al. 2003). Further, outdoor exercise among children who live in areas with high levels of ozone has been associated with an increased risk of asthma development (McConnell et al. 2002). Initial analyses suggest that although walkable areas may have lower levels of ozone and therefore provide multiple health benefits, they may also promote higher exposures to primary traffic-related pollutants (Marshall et al. 2009). Furthermore, the design of many communities represents a challenge to promoting physical activity because of proximity to pollutant sources near residential and recreational areas, lack of sidewalks, and long commuting distances. Given this apparent paradox, it is imperative to better understand the interaction of exercise and poor air quality on cardiorespiratory health and function.

Much attention has been paid to "smart growth" that reduces community dependence on the automobile and, in so doing, promotes physical activity and reduces pollutant emissions. The important public health impacts related to air pollution exposure suggest a need for even smarter growth that focuses on health promotion while also considering air pollution exposure. Examples include active transportation "green" corridors that are separated from major traffic arteries, design of neighborhoods that are both walkable and high density, serving communities with mass transit, and incentives to reduce emissions in urban centers. Further, the development of urban design and transportation planning tools that incorporate health-promoting attributes and that reduce individual-level exposure during travel are needed (e.g., Su et al. 2010; University of British Columbia 2010).

Nutrition

In addition to increasing physical activity, dietary changes can reduce the risk of disease development and therefore reduce susceptibility to the effects of air pollution. For example, consuming diets high in omega-3 fatty acids, even at levels as low as one fish meal per week, has been associated with lower mortality risk from CHD (Marchioli 2003). Although findings regarding supplementation are not entirely consistent, long-term supplementation with omega-3 fatty acids has also been shown to reduce the likelihood of nonfatal myocardial infarctions and stroke, as well as the risk of all-cause, cardiovascular, and sudden deaths (Marchioli 2003). As with omega-3 consumption, reducing salt intake can also contribute to cardiovascular health; studies suggest a 4–6 g/day reduction is associated with a reduction in systolic blood pressure of about 3.5 mmHg and 7 mmHg among normotensive and hypertensive (≥ 140/90 mmHg) individuals respectively, yielding a predicted avoidance of 240–362 cardiovascular events per 100,000 population over 10 years (He and MacGregor 2002; Qin et al. 2009). Consuming a diet high in plant sterols, combining four groups of cholesterol-lowering components of plant origin (viscous fibers, soy protein, plant sterols, and almonds), was also shown to significantly reduce blood pressure in a longitudinal study of 66 hyperlipidemic subjects (Jenkins et al. 2008).

Although literature regarding the effect of dietary modification on cardiovascular health is plentiful, there remains inconclusive evidence assessing how dietary modification or supplementation can modulate the effects of air pollution. Based on the currently prevalent notion that oxidative stress may play a role in the genesis of air pollution effects, it is logical to consider efforts to increase the body's antioxidant defenses as potential interventions to ameliorate the negative health impacts of pollution exposure. Research investigating antioxidant supplementation suggests that intake of vitamins C and E can reduce the effects of ozone on lung function and nasal inflammatory cytokine production in both healthy and asthmatic populations (Chatham et al. 1987; Grievink et al. 1999; Samet et al. 2001; Sienra-Monge et al. 2004; Wiser et al. 2008). Omega-3 fatty acids, which can be found in oily fish such as mackerel and salmon and in flax seed oil, have also been studied because of their potential antioxidant benefit and ability to modulate the oxidative response to pollution. In a study of 52 older adults, daily supplementation with fish oil compared with soy oil reduced the effects of PM2.5 on superoxide dismutase activity, plasma glutathione, and heart rate variability (Romieu et al. 2005, 2008b). Similarly, analysis of supplementation with vitamins B6 or B12 or methionine in 549 elderly men as part of the Normative Aging Study suggests an attenuation of the effects of PM2.5 on heart rate variability (Baccarelli et al. 2008).

Taken together, these data suggest that supplementation with antioxidants such as vitamins C and E and omega-3 fatty acids, as well as vitamins B6 and B12 and methionine, can mitigate selected cardiovascular and respiratory impacts of ozone and PM. Furthermore, a limited body of research suggests that consuming omega-3 fatty acids, reducing salt intake, and having a predominantly vegetarian diet can reduce the baseline risk for the development of cardiovascular disease and could thereby reduce susceptibility to air pollution. Individuals who are considered at special risk of air pollution effects and those who wish to take positive action should be encouraged to follow more general dietary recommendations and increase consumption of fruits and vegetables. Specific recommendations regarding individual supplements and their dosing require substantial additional research.

Medication

Although one study has indicated that drugs such as statins can abrogate an effect of PM exposure on heart rate variability in a subset of [glutathione S-transferase M1 (GSTM1) null] participants (Schwartz et al. 2005), the limited body of evidence is not sufficient at this point to justify specific recommendations for use of statins in relation to air pollution. Still, there are suggestions that optimal therapeutic management may protect individuals with inherent predisposition to altered heart rate variability from the effects of PM pollution: In a European study, evidence of altered heart rate variability in relation to PM exposure was strongest among study subjects not using beta-blocker medication, whereas effect modification was not evident for use of other medications (angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, and statins) (de Hartog et al. 2009). Although the value of therapeutic management in mitigating the effects of pollutant exposure is worthy of further study, as are the potential benefits of dietary modification and supplementation, at present following evidence-based guidelines for primary and secondary prevention of cardiovascular disease is a reasonable approach to lowering the underlying baseline risk or air pollutant impacts.

Modifying Activity Time, Location, and Level to Reduce Dose

Air quality advisories and ongoing air quality health information programs typically recommend changing the timing, location, duration, or intensity of outdoor activity to reduce short-term exposure and effective dose (Environment Canada 2010; U.S. Environmental Protection Agency 2010b), whereas optimal location of home, work, and school is the primary focus of advice related to long-term exposure (U.S. Environmental Protection Agency 2010a).

Air pollution dose is determined largely by the pollutant concentration and inhalation rate. Dose can be expressed over varying time windows (duration). Long-term exposures have been linked to increased risk for cardiovascular mortality (Dockery et al. 1993), whereas short-term exposures to peak pollutant levels have been associated with detrimental respiratory and cardiovascular effects. For example, a number of studies indicate that the incidence of ischemic heart disease events increases with recent exposure, perhaps even within 1–2 hr of exposure (Brook et al. 2010). Less evidence is available on the mechanistic effects of extremely short exposures in the range of minutes to hours. Extremely short-term exposures to high PM levels can occur in many situations, including in traffic jams, at bus stops, in indoor parking garages, and at fireworks displays (Dales et al. 2009; Glorennec et al. 2008; Singh et al. 2010). Short-term exposure to diesel exhaust (1–2 hr) significantly reduces brachial artery diameter in healthy subjects and exacerbates exercise-induced ST-segment depression in people with preexisting coronary artery disease (Mills et al. 2007a; Peretz et al. 2008). Exposure to PM may also alter pulmonary neural reflexes and lead to changes in heart rate variability through an increase in sympathetic stimulation, ultimately leading to arrhythmia (Brook et al. 2004). Although short-term exposure to PM has acute vascular and neural consequences, it is unclear how short-term exposures and specifically, repeated short-term exposures, affect health over the long term. At present there is insufficient data to assess the link between extremely short-term peak exposures and specific mechanisms of action leading to health outcomes. This important research gap makes it difficult to assess whether observed health effects are due to extremely short-term exposure to high concentrations of pollutants or to the average exposure over ≥ 1 days. An improved understanding of this relationship would inform better health recommendations, and messaging may need to be different depending on whether daily, 3-hr average or shorter peak exposures are the exposure duration of greatest consequence to health.

Pollutant levels may vary with time, depending on their sources. For example, fine PM levels may be higher during wintertime evenings in areas affected by residential wood burning, whereas ozone levels tend to be highest during summer afternoons, and traffic-related pollutants peak during rush hours. Human activity also has temporal patterns (Leech et al. 1996), and acute personal exposure to outdoor pollutants is greatest when peak times of outdoor activity correspond to peaks in ambient concentrations, as in the case of ozone in the afternoon (Liu et al. 1997). Personal exposures may be reduced severalfold by avoiding outdoor activity at peak times of day, although this may sometimes mean scheduling activity very early in the morning or deferring activity altogether on advisory days, depending on the pollutant (Campbell et al. 2005).

As described earlier, pollutant concentrations also vary in space. Individuals can reduce their exposure and the risk of adverse health effects by engaging in outdoor activity away from traffic (Kaur et al. 2005; McCreanor et al. 2007). Considering that some health effects, such as myocardial infarctions, may be associated with very short-term exposures, time spent in microenvironments where "high" exposures exist even for short durations, such as in a car during heavy traffic, can also be important (Mills et al. 2007b). Increased activity level increases inhalation rate up to severalfold, and those with higher activity levels in relation to work or recreation outdoors will receive a higher effective dose of pollution, particularly on days when outdoor air quality is poor. Reducing outdoor physical activity on days when air quality is particularly poor can decrease an individual's air pollution dose, but it also reduces exercise levels. Although initial review of the literature suggests that beneficial aspects of active transportation outweigh any negative impacts related to increased air pollution exposure (de Hartog et al. 2010; de Nazelle and Nieuwenhuijsen 2010; Reynolds et al. 2010), further research is needed to better understand the health impacts of increased air pollution exposure during outdoor exercise; this will provide more balanced recommendations for individuals, which take into account the resulting benefits and risks. In addition, preliminary research suggests opportunities for planners that facilitate active transportation, without leading to increased air pollutant emissions or exposures (Su et al. 2010; Thai et al. 2008).

Given that most exposures, even to ambient pollution, occur indoors and that individuals may choose to remain or exercise indoors on days when outdoor air quality is poor, it is important that information on indoor air quality be included in health protection advice regarding air pollution exposure. Besides environmental tobacco smoke, which leads to well-documented exposures and effects, indoor sources such as cooking can generate high concentrations of PM indoors both in residences and commercial settings (Levy et al. 2002). The health impacts related to exposure to indoor-generated PM not related to smoking have not been thoroughly evaluated. Not surprisingly, following advice to stay indoors can reduce exposure to some pollutants while increasing exposure to others (Stieb et al. 2008). There is large variation in indoor:outdoor ratios for pollutant concentrations, both between and within homes. Indoor:outdoor pollutant ratios depend on numerous factors, including the type of pollutant, city, indoor and outdoor sources, building design, use of windows, age of the building, and season. Infiltration efficiency is the fraction of outdoor pollution that penetrates indoors and remains suspended and can be decreased by modifying penetration (the movement of outdoor pollutants to indoors), deposition (the depositing of pollutants on room surfaces), and exfiltration (the movement of pollutants to outdoors). Specifically, decreasing air exchange within a home or building can effectively reduce infiltration. Air conditioning and its coincidence with closing of windows, and the winter season (when windows are also generally closed) all function to reduce infiltration of ozone and PM by reducing air exchange (Allen et al. 2003; Liu et al. 1995; Wallace and Williams 2005).

Indoor pollutant exposure can also be lowered through the use of air cleaners. Several studies have shown that high-efficiency particulate air (HEPA) filter air cleaners can effectively reduce indoor PM concentrations resulting from both indoor (Batterman et al. 2005; Cheng et al. 1998; Green et al. 1999; Offermann et al. 1985) and outdoor (Barn et al. 2008; Brauner et al. 2008; Henderson et al. 2005) sources. However, these studies show that air cleaner effectiveness will differ within and between buildings depending on factors such as air exchange, the capacity of the air cleaner, and baseline pollutant levels. Clinical studies investigating the health benefits of air cleaner use have shown mixed results. A limited number of studies suggest that air cleaners can provide some health benefits by reducing exposure to PM that subsequently trigger biological responses associated with air pollutant exposure (Sublett et al. 2009). Some associations have been found between the use of HEPA filter devices and a reduction in asthma symptoms among adults and children (McDonald et al. 2002) and cat allergy–related symptoms among adults (van der Heide et al. 1997) associated with indoor-generated pollution and allergens. Researchers have also found associations between the use of portable air cleaners and decreased symptoms relating to exposure to outdoor generated pollution (Mott et al. 2002). In one study of elderly persons living in close proximity (< 350 m) to major roads, the use of two portable HEPA filter air cleaners over a 48-hr period was shown to decrease the impact of outdoor-generated PM on microvascular function (Brauner et al. 2008). In contrast, other studies have found no association between air cleaner use and air pollution–related health effects (Blackhall et al. 2003; Warburton et al. 1994; Wood et al. 1998). In a recent review, researchers suggested that investigating the health impacts of air cleaner use over a short-term period (days to weeks), as is the case for most studies, may not allow sufficient time to detect any resulting health benefits (Sublett et al. 2009). In addition to air cleaning, the use of air conditioning has been linked to some reduction in health impacts related to air pollution such as a decreased risk of cardiovascular hospitalizations in communities with a higher prevalence of air conditioning (Bell et al. 2009). The role of air conditioning is presumably related to reduced pollutant infiltration due to the decreased air exchange rates during the use of an air conditioner (because windows are closed), but the above ecologic association may also result from regional and/or socioeconomic factors and may not be specifically linked to air conditioner use (Vedal 2009).

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