A conversation with UC Davis Environmental Health Sciences Center Director Irva Hertz-Picciotto
Irva Hertz-Picciotto, M.P.H., Ph.D. received the International Society for Environmental Epidemiology’s Goldsmith Lifetime Achievement Award in 2011 for sustained and innovative contributions to the field. The UC Davis public health sciences professor has served as principal investigator for several leading federally funded epidemiological studies, including some investigations considered the first or largest of their kind. Excerpts:
Q. How much of a role do you think environmental factors are likely to play in some of the major health problems of our era?
A. When we talk about diseases that have been rising rapidly in recent decades, it’s likely that something has been changing to account for those increases, and that those changes are environmental. While we’ve shown that changes in diagnostic definitions or increased awareness may account for some increases in, say, autism rates, it’s clearly not the full answer. In fact, we and others have shown that only about half of a 600 percent increase in autism over one snapshot 11-year period resulted from diagnostic changes. That leaves a 300 percent increase to explain. It’s also clear that heritable gene mutations alone can’t account for large increases in asthma, autism, ADHD, obesity and diabetes, because these mutations occur rather slowly.
Q. What factors could be playing a role?
A. When we speak about “environment” it’s in a broad sense: nutrition, medications, chemicals, workplace exposures, neighborhood characteristics and so forth. Traditionally, concerns about chemicals were directed primarily at air and water pollution, which still cause a great deal of disease and disability. But we’ve also seen an explosion of new chemicals in our daily lives – in our food (plastic packaging, high fructose corn syrup), cosmetics (lead in lipstick), furnishings (flame retardants), flooring (plasticizers), personal care products (phthalates), and on and on. Many newer chemicals affect brain, endocrine and immune functioning. The quick pace at which the chemicals are introduced has meant the slowness of science in identifying direct links between exposures and potential health impacts can’t keep up.
The problem is exacerbated because regulatory systems are inadequate to the task of protecting the entire population – not just adults in general, but pregnant women, children and fetuses. Despite overwhelming evidence that lead harms children, millions are still unprotected. Meanwhile, thousands of chemicals have yet to be tested for long-term or chronic effects in people, because the burden of proof for chemicals (other than medications) is on the public to demonstrate harm – rather than on the manufacturer to demonstrate safety before introducing them into the market.
Q. When does a connection or link become significant in regard to the overall pursuit of causes?
A. Epidemiologists have spent decades discussing this very question in order to address the structural challenges faced by the field and ensure that our conclusions are, and remain, robust. A drug can be tested by randomly assigning who gets it versus the placebo. Not so for most environmental exposures, largely for ethical reasons. Our studies are generally purely observational – for example, comparing those living near freeways to those living farther away – and they form the basis for much of our public health efforts. For example, almost all knowledge about the human toll of cigarette smoke or of traffic-related air pollution comes from well-conducted observational studies.
Epidemiologists seek rigor at many steps. Before concluding that an association is causal, we have to rule out alternative explanations. These include the possibility that the exposed are systematically different from the unexposed, and that those differences may be the real causes of disease. People living near freeways may be less wealthy, for example, with less health care access or less-healthy diets. These variables would be “confounders” and would have to be controlled in the analysis.
Even if an association were still present after controlling for other causes, this alone would still not be enough to say that the link was “causal.” About 50 years ago, epidemiologists were among those asked (by the U.S. Surgeon General) to determine whether smoking caused lung cancer. Sir Bradford Hill, a leading epidemiologist and statistician, developed a series of “viewpoints’’ that are widely used to help evaluate evidence for causality. One example is “plausibility” – the identification of a mechanism, often through toxicologic studies or molecular biologic experiments, indicating a pathway known or highly likely to occur in humans. So our center also includes this kind of research.
Q. How might health providers consider and use our findings?
A. Exposures affect virtually all organ systems, and the public looks to doctors and nurses for prevention advice, so there’s a real opportunity to educate. Counseling patients on healthier living may also protect them in more ways than we know. For instance, many exposures associated with autism, though not yet fully established as causal, are already accepted as causes of other adverse outcomes – such as neural tube defects (low folic acid), neurocognitive deficits (certain pesticides), and respiratory problems (air pollution).
Even with increased personal awareness, it can still be difficult for individuals to reduce their exposures – whereas regulatory changes can affect entire populations. So health providers also have an essential role, as individuals or through professional organizations, in educating policymakers as well.
Q. One of the center’s goals involves helping to develop preventions.
A. Leading a healthy life isn’t easy, and it’s even more challenging for those with fewer resources. It’s also a burden most individuals don’t have time for. If you wanted to avoid phthalates – a class of chemicals that can interfere with sex hormones – you’d have to read the label of every soap, shampoo, lotion, deodorant or scented product. You’d need to know that “parfum” or “fragrance” is code for phthalates used in scent delivery. And even then there are phthalates from other sources.
In short, for vast numbers of chemicals, high-level legislation is needed to reduce exposures. Since manufacturers often aren’t eager to change products, it takes public pressure to bring about systemic changes. Ultimately, we need a more efficient and effective regulatory system that doesn’t require years or decades to regulate each individual chemical.