Journal Issue: Children with Disabilities Volume 22 Number 1 Spring 2012
Tools for Preventing Harmful Exposures
Policy makers and other public health advocates can take several approaches to preventing disabilities that result from harmful environmental exposures. These are often classified into "the Three Es": education, enforcement, and engineering. Education involves giving people information on health risks in an effort to change their behavior. Enforcement uses legislation and regulations to reduce or curtail harmful behaviors. Engineering involves manipulating the environment to passively reduce exposures to a hazard. For example, to prevent childhood obesity or type II diabetes, children might be given lectures or promotional materials about the risks of a diet high in saturated fat and the benefits of eating more fruits and vegetables (education); fast-food advertising aimed at children might be restricted (enforcement); and neighborhoods might be designed to encourage walking and other physical activity or making healthful snacks and water more easily available to school children than unhealthful ones (engineering).
All three methods have strengths and weaknesses. Education is the least invasive, but changing behavior through education is notoriously difficult and often ineffective (smoking-cessation programs and campaigns aimed at increasing fruit and vegetable consumption tend to have low success rates), especially when modifying one's behavior requires acting differently from friends, family, or the larger society. In contrast with education-only efforts, enforcing certain behaviors, by restricting the sales of tobacco products and alcohol to minors, for example, has been more effective. Enforcement can be quite contentious when it involves regulating industries or people's behavior and often leads to accusations of paternalism or heavy-handedness, although paternalism may be more acceptable where children are concerned. From a population-wide perspective, the engineering approach has the greatest potential to improve health: by making more healthful lifestyles the "path of least resistance," it bypasses the difficult process of persuading people to change their behavior. Engineering the environment, such as treating water to reduce diarrheal diseases, phasing out the use of leaded gasoline to prevent lead poisoning, or instituting zoning codes to limit proximity of residential dwellings to industries emitting toxic material, have all proved to be highly effective ways of preventing disease and disability. At the same time, engineering solutions can be costly to implement. This approach also requires the involvement of professionals outside the typical conception of "health"—engineers and city planners, as well as political leaders—in addition to physicians and public health scientists. Still, as noted, some of the largest increases in life expectancy over the past century have resulted primarily from population-wide engineering solutions.81
Population-wide approaches to prevention can be effective because they are capable of "shifting the curve." Disabilities exist on a continuum. Thus, a small increase in risk for a common disease or disability affects population health more than a large increase in a rare condition.82 For example, children's capacity for attention, hyperactivity, and impulse control varies across a wide spectrum, and it is only to simplify the diagnosis and treatment that health care providers create a clear division between "normal" children and those who have ADHD. Geoffrey Rose used the idea of "shifting the curve" to describe the relationship between individual- and population-level risks. He showed that many diseases or disabilities exist on a continuum; the number of people in the "high-risk" group (in this case, those corresponding to the clinical criteria for ADHD) is closely tied to the overall state of the population as a whole.83 In other words, the number of children diagnosed with ADHD in a population can often be predicted from the average behavioral profile of children in the population. Depending on the shape of the distribution, small shifts in behaviors or exposures associated with ADHD can have a dramatic effect on the number of children who meet clinical criteria for ADHD. And in practice, with the exception of immunizations, population-wide interventions to prevent disabilities are largely limited to modifying environmental risk factors.
Taking the Precautionary Principle with Children's Toxic Exposures
How much evidence is needed before action is taken? The dangers of tobacco and lead were understood for decades before prevention became a priority. Today, however, for a variety of reasons, policy makers are reluctant to act on a hazard unless the precise way that it causes disease or disability is known.84 The sanitarians of the early twentieth century understood that demonstrating a pattern of disease was sufficient to take action, often decades before the bacterial causes were discovered. One way to apply that lesson is by reforming the way industrial chemicals are tested and allowed onto the market.
Currently, industrial chemicals are "innocent until proven guilty." They can be introduced without being fully tested for toxicity: indeed, of more than 200 industrial chemicals known to have neurotoxic effects in adults, only a handful have been tested for neurotoxicity at lower (subclinical) doses.85 Moreover, a chemical is taken off the market or a pollutant regulated only when harmful effects are proven definitively; by convention, this means that a chemical has to be proven toxic in laboratory experiments and then in a series of epidemiologic studies, which usually take decades to complete.86 In the interim, millions of people, including children and pregnant women, will have been exposed and possibly harmed. Thousands of chemicals are currently in the environment, making it difficult to attribute disability or disease to any one particular chemical. For those chemicals that persist indefinitely in the environment, even when harmful effects are identified, stemming the tide of exposure may be the most that can be accomplished. Although the insecticide DDT was banned in the United States in the early 1970s, one recent study estimated that its metabolite DDE can be detected in 95 percent of Americans.87 It has been linked with diabetes, spontaneous abortion, and impaired neurodevelopment.88 PCBs, which have been linked to reduced IQ and immune system and thyroid dysfunction, have been banned for decades; however, they are routinely detected in newborns and children around the world; exposure is nearly universal.89
The experience with lead, tobacco, PCBs, mercury, and other toxicants indicates that the United States should adopt the precautionary principle and identify toxicants before they are marketed and widely disseminated in the environment. Other governments have already taken such a step. In 2007 the European Union instituted the REACH Program, which requires manufacturers to prove that chemicals are safe before they are marketed.90
Setting Priorities: Population Attributable Fractions
How do we prioritize what environmental influences or risk factors to target? From a prevention perspective, efforts should focus on common and modifiable risk factors associated with high-prevalence disabilities and potentially debilitating conditions, such as ADHD, obesity, or asthma. A tool known as population attributable fraction, a measure of the proportion of disability or disease in a population that can be attributed to a particular risk factor, can help quantify priorities.91 The population attributable fraction takes into account both the risk posed by an exposure and the frequencies of exposure and disease in the population.
Tanya Froehlich and her coworkers estimated that exposure to higher levels of lead and prenatal tobacco each accounted for 500,000 additional cases of ADHD in U.S. children; using the population attributable fraction, they estimated that 38 percent of cases of ADHD could be prevented if childhood lead exposure and smoking in pregnant women were eliminated.92 They also showed that, because lead and tobacco interact synergistically, children who had high blood lead and exposure to prenatal tobacco constituted only 7.7 percent of the population, but they represented nearly 25 percent of the total cases of ADHD.93 It is worth noting that both blood lead levels in children and smoking among pregnant women have decreased significantly in the last few decades, so they cannot explain the increase in ADHD prevalence.94 However, the prevalence of ADHD would undoubtedly be higher if these two environmental factors had not been reduced. There are now several other toxicants, as well as other risk factors, suspected of contributing to the development of ADHD.95 However, the current health care system continues to focus almost entirely on identification and treatment of children for ADHD rather than on further reductions in toxicants demonstrated or suspected of elevating the risk for the disorder.
Calculating population attributable estimates for prevalent disabilities is not always feasible —it requires a representative sample, an estimate of the prevalence of exposure, and a measure of risk—but estimates do exist for several notable risk factors for prevalent childhood disabilities (table 1). Bruce Lanphear and others estimated, for example, that residential exposures—including exposure to secondhand smoke, the presence of pets, use of a gas stove, and allergies to dust mites or cockroaches—accounted for approximately 533,000 cases of asthma (39 percent of all cases) in children under six and 2 million cases (44 percent of the total) in children aged six to sixteen.96