Published on 8 March 2022
News
Combating climate change is an opportunity to improve health
Rémy Slama is an environmental epidemiologist and Director of Research at Inserm. He studies the influence of environmental contaminants (air pollutants, endocrine disruptors, exposome), particularly in the context of early exposure, on human health.
He has been invited for 2021-2022 to hold the Public Health Chair, created in partnership with Santé publique France.
Environmental health research studies the external factors responsible for certain diseases. What are these factors?
Rémy Slama: Broadly speaking, the Anthropocene, which can be traced back to the Industrial Revolution at the end of the 18thcentury in Europe, marks a break with the past. Before the 19thcentury , most causes of death were linked to infectious agents and famine. With the Industrial Revolution, our European societies underwent an epidemiological transition that brought infectious diseases under tight control. Life expectancy increased, and the main causes of death became chronic diseases, such as cardiovascular and neurodegenerative diseases and cancers. Environmental health research focuses on the more distant causes of these illnesses - the causes of death, as it were. These take the form of physical, chemical, behavioral, social and, to this day, infectious factors, although the latter no longer represent the main contribution. All this makes up the exposome, a concept that scientists have been pondering for the past fifteen years. It refers to all the environmental exposures we undergo from conception to the end of our lives. We have a partial vision of the influence of this exposome on health. Some factors have a strong and fairly precisely quantified influence, such as smoking, alcohol, sedentary lifestyle, dietary imbalances and air pollution. Other factors, such as lead, mercury, radon, benzene and certain persistent organic pollutants, which have been banned but to which we are still exposed, have a definite influence on a smaller scale. For many other factors whose exposure is frequent in the general population, either the level of evidence concerning a health effect is weaker, or we are not at the stage of precisely quantifying the population impact in terms of the number of cases of pathologies attributable to exposure. This is the case for certain endocrine disruptors such as bisphenols.
When was the influence of environmental factors on certain diseases first identified?
It's been suspected for a very long time. As early as the5th century B.C., Hippocrates explained that, on arriving in a city, a physician must take into account the local climate, diet and activities of its inhabitants in order to understand and treat the diseases that affect them. Later, a few visionary scientists identified the effect of certain factors. The 15th-century physician-surgeon Paracelsus is regarded as the father of toxicology, and in18th-century France, Louis Villermé, the precursor of sociology, identified the major contribution of social inequalities to mortality. But the real rise of toxicology and environmental epidemiology, the two central disciplines of environmental health research, took place in the second half of the 20th century. For epidemiology, this corresponds to the development of the cohort approach, which first made it possible to identify the role of risk factors such as tobacco, alcohol and lack of physical activity, easily characterized by simple questionnaires. The development of toxicology and the sharp increase in the sensitivity of biomarkers of human exposure at the beginning of this century have enabled us to characterize an ever larger part of the exposome and investigate its health effects and mechanisms of action.
With regard to this characterization: are there still social inequalities in exposure to these environmental factors?
Absolutely. On the one hand, there are social inequalities in exposure, and on the other, major social inequalities in health. As regards the former, the link between social factors and environmental exposure is complex and varies from one family of substances to another. This field of research can be referred to as environmental justice by the social movement associated with it. If we distinguish environmental factors according to their main source of variation, we can consider that factors with strong spatial contrasts - proximity to polluting industrial sites, major roads, the presence of green spaces... - are subject to significant social gradients, with exposure to levels deleterious to health being more frequent in less-favoured social categories. The situation is less unequivocal for substances whose levels depend above all on behavior, as the behaviors that determine exposure vary in complex ways across the population. For example, more affluent populations tend to be more often exposed to persistent pollutants or metals present in fish, whose consumption also follows a social gradient, while exposure to lead, another metal, is more frequent in disadvantaged populations.
What methods can epidemiologists use to assess the link between an environmental factor and a disease?
With laboratory animals, we can experiment, i.e. control exposure and monitor its biological effects. This experimental approach is used in humans to test the efficacy of drugs, but very rarely, for ethical reasons, for potentially dangerous substances. One of the challenges of environmental epidemiology is therefore to make causal inferences from non-experimental, i.e. observational, data. This context increases the risk of confounding bias and exposure measurement errors. A first approach consists of recruiting subjects while they are disease-free, quantifying exposures, for example via biomarkers, and following populations long enough for a certain number of cases to eventually occur, in order to see whether exposure is associated with an increased risk of disease occurrence. This cohort approach is rather long - depending on the pathology studied - and requires many subjects, a high proportion of whom will not develop the disease of interest. An alternative is to recruit cases of the disease of interest directly, in hospitals for example, and compare them with unaffected subjects, the controls. This is the logic of the "case-control" approach, which is faster and less costly than the cohort, but very limited by its retrospective nature, particularly in the context of factors that are not very persistent in the organism. In all cases, confounding factors are controlled by a statistical approach or by the study methodology. Cohort and case-control approaches are typically used to identify hazards - demonstrating whether a factor can cause a certain type of effect. When it comes to quantifying impact, i.e. the number of cases of a pathology attributable to environmental factors, quantitative health impact studies are used; they are used, for example, to try to anticipate certain consequences of climate change.
How is environmental health affected by global warming?
The frequency of extreme climatic events (floods, fires, etc.), temperature variations and changes in the range of certain infectious disease vectors are all manifestations of climate change that have an impact on human health. To achieve carbon neutrality, we need to take action in sectors such as agriculture, transport, energy production and housing, which are the main sources of greenhouse gas emissions. Yet all of these are, at various levels, linked to our health - via diet, physical activity, air pollution, etc. Although climate change itself is a threat, combating it is an opportunity to improve health. By limiting greenhouse gas emissions, we could combat certain major public health problems linked to unbalanced diet, sedentary lifestyles and atmospheric pollutants. Current evidence suggests that health co-benefits can be expected in many of these areas. This new line of research illustrates one of the beauties of our field of research, linked to its profound multidisciplinarity. We work with climatologists, physicists and chemists to characterize environmental factors and human exposure; with molecular biologists and toxicologists to identify biomarkers of effect, and understand the mechanisms of action of certain substances; with clinicians to set up medical follow-up in our studies; with data science specialists to identify approaches suited to our complex study designs, generating a colossal number of variables. And, of course, with society, which is concerned about the impact of environmental factors.
Does the identification of a risk factor systematically lead to its regulation?
Research by environmental health specialists aims to identify health hazards, understand their biological mechanisms, quantify the attributable risk on a population scale and, increasingly, evaluate different options for managing the problem. So, beyond the cognitive component, the aim is to nurture risk management. The logics and strength of management vary from one risk factor to another, for reasons that go well beyond science. These logics include authorizations set according to "dose limits", for air pollution for example; prohibition, as is the case for persistent organic pollutants and asbestos; or even more economic approaches, as with tobacco and as we are moving towards for greenhouse gases. The maximum level of regulation corresponds to the international convention - for example, the Stockholm Convention on persistent organic pollutants such as DDT and PCBs - which prevents hazardous substances banned in rich countries from being used in other, less wealthy countries. Historically, our societies have tended to wait for absolute certainty before acting on a factor harmful to health, without always making the effort to generate new knowledge. The harmful effects of lead have been suspected, if not known, for over two thousand years, but it was not until the 20th century that strong regulations were introduced. In fact, these regulations were introduced sector by sector, and lead is still widely used around the world. The precautionary principle is supposed to provide a framework that allows - and even encourages - action in situations of uncertainty, while encouraging the generation of scientific knowledge to reduce this uncertainty. However, in many cases, without even mentioning precaution, the logic of prevention is difficult to understand: fine particles, which have been shown to cause cancer and have a major impact on health, have a dose limit in Europe that is five times higher than that recommended by the World Health Organization (WHO). In addition to the difficulty of dealing with uncertainty, our societies are faced with the challenge of multitude. There are tens of thousands of chemical substances on the market - twenty-three thousand marketed in excess of one tonne per year, the current declaration threshold - and managing this multitude is complex. Management by major categories of health or environmental hazards (carcinogens, mutagens, endocrine disruptors, etc.) provides a way of avoiding the need for decision-makers to decide on a substance-by-substance basis. It's up to society to decide how to protect itself from each hazard. It's up to regulatory agencies to classify substances into hazard categories.
On an international scale, where does France stand in relation to these regulations?
A large proportion of French regulations on environmental health issues are linked to European directives, which is not to say that we are subjected to them - Europe is made up of its members, there is a dialogue between the European and national levels, and sometimes the European Union generalizes interesting initiatives taken by a member country acting alone, which can of course be France. In the case of endocrine disruptors, for example, France has been a driving force, and some of the decisions we have taken - such as the ban on bisphenol A in food containers - have subsequently been adopted in part at European level. All in all, I'd say that we have relatively advanced and protective chemical substance regulations in Europe compared to other industrialized countries. Endocrine disruptors and carcinogenic substances, for example, are banned in pesticides, which is not the case in the USA or Japan. On the other hand, in another area - air quality - we are lagging far behind the United States, which banned leaded petrol and smoking in public places long before we did, and which has standards for fine particles corresponding to around half the European threshold for the annual average. These major discrepancies in decision-making on dangerous substances between countries with similar standards of living cannot, of course, be justified scientifically, and illustrate the limited place of science in political decision-making.
Interview by William Rowe-Pirra
