Environmental diseases
Environmental diseases refer to health conditions that arise due to exposure to adverse physical, chemical, biological, or social environmental factors. The severity and outcomes of these diseases can vary, with acute cases leading to rapid health deterioration and chronic cases stemming from prolonged exposure to lower levels of harmful agents. Historically, the concept of miasma illustrated the belief that poor environmental conditions could lead to disease, a notion still echoed in modern understanding.
Research has established connections between environmental quality and disease through rigorous epidemiological studies, which often consider various factors such as age, sex, and genetics that may influence individual susceptibility. Notably, guidelines developed by figures like Austin Bradford Hill provide frameworks to explore causal relationships between environmental exposures and diseases, emphasizing the importance of consistency, correlation, and plausibility in establishing causation.
The symptoms of environmental diseases can be acute, requiring immediate intervention, or chronic, often developing over many years due to environmental pollutants. Treatment strategies focus on minimizing exposure and may include therapies tailored to specific toxic agents. The current global landscape highlights the urgent need for effective public health policies and interventions, particularly as environmental risk factors contribute to a significant portion of the global disease burden, underscoring the critical intersection of environmental health and human well-being.
Environmental diseases
ANATOMY OR SYSTEM AFFECTED: All
DEFINITION: Sicknesses caused or exacerbated by human exposure to physical, chemical, biological, or social environmental conditions, the duration and intensity of the exposure typically affecting the manifestation of symptoms and fatality-case ratios. Acute environmental diseases may result in rapid decline of health status and warrant emergency response, while chronic conditions often result from long-term exposures to low levels of environmental risk factors.
Causes and Symptoms
The modern word “miasma” comes from the Greek miasma or miainein, meaning “pollution” or “to pollute.” Before scientific theories of disease became entrenched in medical practice, miasma was used to connote bad environments in which human exposure led to various diseases. Even today, one of the most devastating human diseases, malaria, draws its name from references to “bad air.” There is clearly a rich historical record of human recognition of the intimate connection between environmental quality and diseases. It is now known that serious human diseases are caused by numerous chemical, physical, and biological agents (risk factors) that occur naturally or as a result of human actions that modify the environment. In fact, the more that is learned about disease etiology, the more the complex interplay between environmental conditions and root causes of diseases within the body are recognized. Furthermore, some people are more sensitive to environmental because of their age, sex, occupation, culture, or genetic characteristics.
Environmental diseases are those illnesses for which cause and effect can be reasonably associated through epidemiological studies, preferably verified through laboratory experiments. Therefore, the recognition of environmental diseases draws upon two traditional postulates regarding causation in the study of human diseases, one ascribed to Robert Koch (1843–1910) and the other ascribed to Austin Bradford Hill (1897–1991). The more important set of guidelines for environmental diseases is generally known in epidemiology as Hill’s criteria of causation, based on his landmark 1965 publication entitled “The Environment and Disease: Association or Causation?” Hill warned that cause-effect decisions should not be based on a set of rules. Instead, he supported the view that cost-benefit analysis is essential for policy decisions on controlling environmental quality in order to avoid diseases. It is arguable that Hill’s treatise initiated current trends characterized by the precautionary principle in environmental health science. Nevertheless, Hill’s nine viewpoints for exploring the relationship between environment and disease are worth emphasizing. They are precedence, correlation, dose-response relationship, consistency, plausibility, alternatives, empiricism, specificity, and coherence.
According to the precedence viewpoint, exposure must always precede the outcome in every case of the environmental disease. One of the most famous examples here is the classic epidemiological study of John Snow (1813–58) on the spread of cholera and its association with exposure to contaminated water in the densely populated city of London.
According to the viewpoint, a strong association or correlation should exist between the exposure and the incidence of the environmental disease. The clustering of diseases within neighborhoods or among workers at a specific occupation is frequently the beginning of investigations into environmental diseases. Clusters can provide strong evidence of correlations. Bernardino Ramazinni (1633–1714), considered by many to be one of the founders of the discipline of occupational and sciences, published his treatise De Morbis Artificum in 1700 following critical observations regarding the correlation between environmental exposures of and diseases in workers.
According to the dose-response viewpoint, the relationship between exposure and the severity of environmental disease should be characterized by a dose-response relationship, in which an increase in the intensity and/or duration of exposure produces a more severe disease outcome. “The dose makes the poison” is one of the central tenets of environmental toxicology. This phrase is attributed to Paracelsus (1493–1541). This tenet has proven difficult to interpret for formulating health policy in the case of environmental diseases because the variation in human and means that, in many situations, a single threshold of toxicity cannot be established as safe for every person. Exposure to ionizing radiation is an example of a situation in which it is difficult to establish dose-response relationships that are useful for setting uniformly applicable preventive health policy.
According to the consistency viewpoint, there should be consistent findings in different populations, across different studies, and at different times regarding the association between exposure and environmental disease. This means that the relationship should be reproducible. For example, exposure of people to mercury across civilizations, occupations, and age groups has been consistently associated with certain health effects that allowed the recognition of the special hazards posed by this toxic metal. Mercury was used in various manufacturing processes for several centuries, and where precautions are not taken to prevent human exposure, disease invariably results.
Consistency should cut across not only generations but also occupations and different doses of exposure. For example, “mad hatter’s” disease was associated with the use of mercury in the production of fur felt, in which mercurous nitrate was used to add texture to smooth fibers such as rabbit fur to facilitate matting (the process is called "carroting" because of the resulting orange color). More recently, the exposure of pregnant women to fish contaminated with methyl mercury from industrial sources in Japan produced developmental diseases in fetuses. The societal repercussions of the so-called Minamata Bay disease are still not completely settled after more than fifty years. Mercury is now widely recognized as a cumulative toxicant with effects and organ damage, with symptoms including trembling, dental problems, blindness, ataxia, depression, and anxiety.
According to the plausibility viewpoint, compelling evidence of “biological plausibility” should exist that a pathway leads from exposure to a specific environmental risk factor to the development of a specific environmental disease. This does not exclude the possibility of multiple causes, some acquired through environmental exposures and others through genetic processes. For example, lead poisoning has been recognized since the 1950s as a pervasive and devastating environmental disease. The symptoms of vary, from specific organ effects, such as kidney disease, to systemic effects, such as , and to cognitive effects, such as intelligence quotient (IQ) deficiency. How a single environmental toxicant can produce such wide-ranging diseases was a puzzle until the molecular mechanisms underpinning lead and the pharmacokinetic distribution of lead in the human body was understood. Lead is temporarily stored in the blood, where it binds to a key enzyme, aminolevulinate dehydratase, which participates in the synthesis of heme. The by-products of that reaction produce anemia and organ effects, including kidney and diseases. Long-term storage of lead in the body occurs in bony tissue, where other effects are possible. These biological understandings have helped activists and scientists agitate for environmental policy to reduce lead exposure worldwide.
According to the alternatives viewpoint, alternative explanations for the development of diseases should be considered alongside the plausible environmental causes. These alternative explanations should be ruled out before conclusions are reached about causal relationships between environmental exposures and disease. For example, the typically low doses to which populations are exposed to pesticides and the long time period between exposure and the typical chronic disease outcomes, such as cancers and neurodegenerative disorders, make it difficult to reconstruct the disease pathways and pinpoint causative agents. This is where it is important to consider all alternatives and to eliminate them before compelling arguments can be made about the effects of pesticide toxicity. Sometimes observing wildlife response to environmental risk factors help narrow down alternative explanations, as Rachel Carson taught in her timeless book Silent Spring (1962).
According to the empiricism viewpoint, the course of environmental disease should be alterable by appropriate intervention strategies verifiable through experimentation. In other words, the disease can be preventable or curable following manipulation of the environment and/or human physiology. For acute exposures, the emergency response is to eliminate the source of exposure. However, this is not always possible in cases where patients are unconscious or otherwise unable to articulate clearly the source of exposure, as is the case for many children. Nevertheless, standardized procedures exist for responding to environmental exposure beyond eliminating the source. For example, therapy based on chelation (from the Greek chele, meaning “claw”) works for toxic metal exposure because the mode of action of the therapeutic agent, ethylene diamine tetra-acetic acid (EDTA), is well understood. It is possible to establish empirically the relative effectiveness of EDTA in dealing with various forms of toxic metal exposures. For example, under normal physiological conditions, EDTA binds metals in the following order: iron (ferric ion), mercury, copper, aluminum, nickel, lead, cobalt, zinc, iron (ferrous ion), cadmium, manganese, magnesium, and calcium. Based on this information, it is possible to design therapeutic processes that minimize adverse side effects.
According to the viewpoint, when an environmental disease is associated with only one environmental agent, the relationship between exposure and environmental disease is said to be specific. This strengthens the argument for causality, but this situation is extremely rare. For example, the rarity of mesothelioma, a lung disease that afflicts people who have been exposed to asbestos fibers, made it possible to use epidemiological evidence quickly to support policy in restricting the use of in commercial products and to protect employees from occupational exposures.
The recognition of new diseases often leads to speculation about causative agents or conditions. Occasionally, new ideas about causation challenge orthodox theories. According to the coherence viewpoint, it is important to conduct a rigorous of coherence with existing information and scientific ideas before such causes are accepted in the case of environmental diseases. For example, the origin of neurodegenerative diseases associated with exposure to prion protein remains mysterious, and some environmental causes have been proposed, including exposure to toxic metal ions. Another example is the current concern with the introduction of nanoparticles into commercial products, with concomitant environmental dissemination. Although much has been learned from an understanding of the human health effects of respirable particulate matter, researchers should be sufficiently open-minded to the possibility that nanoparticles will behave differently in the environment and in the human body.
Hill’s nine viewpoints were presented in the context of pitfalls associated with overreliance on statistical tests of “significance” as a justification to base health policy on epidemiological observations. Hill’s viewpoints have been debated extensively, and it is worth noting the following caveats presented in the 2004 article “The Missed Lessons of Sir Austin Bradford Hill,” by Carl V. Phillips and Karen J. Goodman: statistical significance should not be mistaken for evidence of substantial association; association does not prove causation; precision should not be mistaken for validity; evidence that a causal relationship exists is not sufficient to suggest that action should be taken; and uncertainty about causation or association is not sufficient to suggest that action should not be taken.
The second set of guidelines regarding causality derives from what is generally known as Koch’s postulates, but it is perhaps only useful for precautionary approaches to proactive assessment of potential health impacts of new agents about to be introduced into the environment. This approach complements the epidemiology-based inferences described by Hill, but further refinement is warranted to deal with complicated issues such as interactions between multiple environmental agents, which could have additive, neutral, or canceling effects. The question of dose is also difficult to subject to simple conclusions because of phenomena such as hormesis, in which small doses may show beneficial effects.
For environmental diseases, a modified version of can be expressed as follows. First, exposure to an environmental agent must be demonstrable in all organisms suffering from the disease but not in healthy organisms (assuming predisposition factors). Second, the identity, concentrations in different environmental and physiological compartments, and transformation pathways of the agent must be known as much as possible. Third, the agent should cause disease when introduced into healthy organisms. Fourth, biomarkers showing modification of the physiological target affected by the environmental agent must be observable in experimentally exposed organisms.
Treatment and Therapy
The symptoms of environmental diseases vary widely, and physiological, anatomical, and behavioral characteristics can succumb to the effects of environmental agents. In evaluating treatment and therapy, it is useful to consider two categories of symptoms. Acute symptoms are exhibited in response to human exposure to high doses of toxic agents within a short period of time. Essentially, the body is overwhelmed, and emergency therapy is necessary to avoid death or permanent disability. For toxic air contaminants, respiratory distress is a common symptom, and can occur rapidly. Conversely, chronic symptoms of human exposures to low levels of environmental (particularly air) pollutants are difficult to diagnose, as in the case of cancers attributable to secondhand tobacco smoke or ambient exposure to respirable particulate matter. Similarly, exposure of the skin to rapidly absorbed toxins can produce rapid mortality, but the development of skin cancer due to ultraviolet (UV) light exposure may take decades to manifest. Ingestion of contaminated liquids or food may take minutes to provoke distress and vomiting, whereas it may take years for chronic symptoms to manifest in cases of carcinogenic water pollutants.
Treatment and therapy of environmental diseases requires accurate diagnosis of the causative agent. The first line of response is to limit exposure through flushing the body with clean air or liquids. Chelation therapy can be used to reduce the body burden of certain toxic metals. Curative measures follow the established procedures developed for specific organs. For example, chemotherapy, radiotherapy, and surgery are used to treat cancers regardless of the involvement of known environmental factors in their etiology. Skin diseases such as chloracne associated with exposure to chlorinated aromatic pollutants, including dioxins and polychlorinated biphenyls (PCBs), are managed to reduce the severity of lesions and enhance natural healing processes. Cognitive deficits associated with exposure to metals and other environmental pollutants are believed to be reversible as long further exposures are avoided. Finally, environmental diseases associated with infectious agents such as can be controlled through a combination of source disinfection and therapy.
Perspective and Prospects
There has been a resurgence of interest in environmental diseases because of societal changes at regional and international levels. Industrialization demands the use of thousands of potentially hazardous chemicals that ultimately end up polluting human environments and remain an important source of causative agents for environmental diseases. Recent threats associated with global environmental change, bioterrorism, and chemical warfare have all contributed to the need for rapid detection of hazardous environmental agents and tougher laws to protect air, water, soil, and food resources. Prevention is still the crucial solution to reducing the human burden of environmental diseases worldwide.
On June 16, 2006, the issued a landmark report estimating that environmental risk factors play a role in more than 80 percent of diseases regularly reported by WHO across fourteen regions globally. The environment has an impact on human health through exposures to physical, chemical, and biological risk factors and through changes in human behavior in response to environmental change at local and global levels. Globally, nearly 25 percent of all deaths and of the total disease burden (measured in disability-adjusted life years, or DALYs) can be attributed to environmental quality. The situation is more dire for children aged fourteen and younger, with environmental risk factors accounting for more than 33 percent of the disease burden. These discoveries have important implications for national and international health policy, because many of the implicated environmental risk factors can be modified by established interventions. The lack of understanding on how to deploy these interventions globally has inspired the involvement of well-funded organizations and institutions in environmental health issues. According to WHO in 2023, air pollution is responsible for 6.7 million premature deaths each year.
Bibliography
Babatola, Samuel Soledayo. “Global Burden of Diseases Attributable to Air Pollution.” Journal of Public Health in Africa, vol. 9, no. 3, 21 Dec. 2018, p. 813, doi: 10.4081/jphia.2018.813. Accessed 3 Aug. 2023
Carson, Rachel. Silent Spring. 50th anniversary ed. London: Penguin Classics, 2012.
"Health Effects of Exposure to Substances and Carcinogens." Agency for Toxic Substances and Disease Registry, March 3, 2011.
Hill, Austin Bradford. “The Environment and Disease: Association or Causation?” Proceedings of the Royal Society of Medicine58 (1965): 295–300.
"Household Air Pollution." World Health Organization, 15 Dec. 2023, www.who.int/news-room/fact-sheets/detail/household-air-pollution-and-health#. Accessed 1 Apr. 2024.
McMichael, Tony. Human Frontiers, Environments, and Disease. New York: Cambridge University Press, 2003.
National Institute of Environmental Health Sciences (NIEHS). "Environmental Diseases from A to Z." 2d ed. Research Triangle Park, N.C.: U.S. Department of Health and Human Services, National Institutes of Health, June 2007.
National Institute of Environmental Health Sciences (NIEHS). "Advancing Science, Improving Health: A Plan for Environmental Health Research—2012–2017 Strategic Plan." Research Triangle Park, N.C.: U.S. Department of Health and Human Services, National Institutes of Health, 2012.
National Toxicology Program.Report on Carcinogens. 12th ed. Research Triangle Park, N.C.: U.S. Department of Health and Human Services, Public Health Service, 2011.
Pruss-Ustun, A., and C. Corvalan, eds. Preventing Disease Through Healthy Environments: Towards an Estimate of the Environmental Burden of Disease. Geneva: World Health Organization, 2006.
Solomon, Gina, Oladele A. Ogunseitan, and Jan Kirsch. Pesticides and Human Health: A Resource for Health Care Professionals. San Francisco: Physicians for Social Responsibility, 2000.