Environmental determinants of disease

DALYs

Disability adjusted life years: the sum of the years of life lost due to premature mortality (YLL) in the population and the years lost due to disability (YLD) for incident cases of the health condition.

Environment

The surroundings and external conditions, especially as affecting human lives.

Environmental justice (EJ)

The notion of equity and justice in relation to environmental exposures and related health burdens born by groups defined on the basis of such factors as socio-economic status, ethnicity and gender.

Definitions

Last (Dictionary of Epidemiology, OUP[1]) defines the environment as 'all that is external to the human host'.  It can be taken to include anything except genetic factors, but is usually thought of as including physico-chemical, biological, social and cultural factors, which individually and in combination influence human health and well-being. The following notes concentrate on the physical, chemical and biological environment, and give less emphasis to broader socio-economic influences.

Environmental justice

It is important to note however that socio-economic factors lie behind many of the large differentials in health within populations and between them.  In relation to the environment, socio-economic deprivation is important as a frequent determinant of environmental exposures (on average poorer people tend to live in more polluted, less healthy areas) and are more vulnerable to the effects of such exposures by virtue of their health status and poorer access to services.  At the same time, people in more deprived areas, who tend to consume fewer resources, are typically responsible for fewer of the emissions that cause environmental contamination. This is often labelled the issue of environmental justice (or injustice). Examples of inequities in exposure and health effects can be found in relation to many environmental factors.  Examples include:

• exposure to road traffic: poorer people tend to have poorer access to cars, but are often more exposed to traffic-related emissions, and more likely to live on main roads where there is cheaper housing; 
• climate change: those in the richer countries are larger per capita contributors to the problem (produce more CO₂), yet the burdens are likely to be greatest in low-income countries.

Trends in environment and health concerns

Until the 1980s, environment and health concerns were often focused on the adverse consequences for health of the increasing release of chemical contaminants into local environments, and on exposures related to industrialisation and urbanisation.  Particular concerns arose in relation to outdoor air pollution, contamination of water bodies and the land, and radiation risks.  These concerns often received prominence with serious pollution episodes, some examples of which are:

• 1952 London smog, which was responsible for probably several thousand deaths in London as a result of the build up of sulphurous air pollution during a period of unusual meteorological conditions (temperature inversion) which reduced the dispersion of air pollutants.
• 1956 episode of organic mercury poisoning in Minamata, Japan, which arose from the release of methyl mercury in the industrial wastewater from a chemical factory, and its bioaccumulation in shellfish and fish in Minamata Bay, which were then when eaten by the local population.
• 1984 Bhopal disaster, in the Indian state of Madhya Pradesh, often considered the world's worst industrial disaster, in which a release of 40 tonnes of methyl isocyanate gas from a subsidiary plant of the Union Carbide company killed several thousand people, many instantly, and injured many more.
• 1986 Chernobyl nuclear accident which resulted in the release of a plume of radioactive dust which drifted over the western Soviet Union (especially the Ukraine and Belarus), parts of Europe and eastern North America.

The occurrence of key environmental incidents and accidents raise public concern and often lead to new trends in research and interest on the effect of the environment on health.

Industrialisation and urbanisation

Industrial development and urbanisation has been a major and continuing global trend. Half of all people now live in urban environments, and urban life is often polluted and unhealthy, especially in low income countries. Large cities in poorer countries typically combine the environmental problems of poverty with those of uncontrolled urban expansion, squatter settlements and unregulated industrialisation.  Residents therefore often face the combined hazards of poor sanitation, unsafe drinking water, precarious housing, dangerous roadways, polluted ambient and indoor air, and toxic wastes.

Global environmental change

In recent years, it has increasingly been recognised that human activity is now giving rise to a set of large-scale environmental problems, with capacity to cause adverse effects at a global scale.  Such activity is reducing biodiversity at an unprecedented speed, depleting non-replenishable resources, and altering the gaseous composition of the lower and middle atmosphere.  Among the more prominent effects with potential bearing on human health are:

• global climate change (see section 3), arising from the emission of carbon dioxide and other greenhouse gases
• stratospheric ozone depletion, arising from the catalytic destruction of atomic chlorine and bromine derived from chlorofluorocarbons (CFC) and other ozone-depleting substances, with consequent reduced protection to harmful ultraviolet-B(UVB) radiation at wavelengths 270 to 315 nm.

Global change should be distinguished from globalisation, which refers to the processes that lead to increasing flows of people, goods, information and ideas across the world.  Globalisation is itself an important influence on health because of its bearing on such factors as the spread of (transmissible) disease, the export from high to lower income countries of occupational exposures, and the economic factors that contribute to health inequalities and unhealthy patterns of production and consumption.

Kuznets curve

For many environmental pollutants, it has been observed that exposures tend to follow an inverted-U trajectory over time as exposures rise and then fall in parallel with increasing wealth, literacy and technological development.  This is sometimes referred to as the 'environmental Kuznets' curve (figure)[2].

Low income populations tend to suffer most from households exposures, such as those relating to lack of access to clean water and sanitation, and exposure to indoor pollution from the inefficient and inadequately ventilated burning of biomass fuels for domestic cooking and heating.

Middle income populations on the other hand suffer more from community-level pollution, such as urban outdoor air pollution, contaminated water sources, and occupational hazards.

High income populations have, for the most part, reduced these risks, but contribute disproportionately to global problems, including greenhouse gas emissions, persistent organic pollutants (POPs), ozone-depleting gases, and urban non-biodegradable waste.

Global burdens of disease

It is difficult to provide definitive estimates of the environmental burden of disease because of issues of definition, incomplete evidence about aetiology, and the complexities of assessing exposures and their longer-term effects.  But estimates suggest that somewhere between a quarter and a third of the global burden of disease and premature death is attributable to direct environmental risk factors.

The international Global Burden of Disease initiative has attempted to derive estimates of mortality and disability adjusted life years (DALYs) for selected causes. The mortality and DALYs attributed to childhood and maternal under-nutrition are given for comparison (table).  For the environment they indicate that the largest burdens globally arise from unsafe water and sanitation and indoor smoke (both mainly affecting populations in the developing world), and outdoor air pollution (mainly affecting middle income populations, especially in Asia).  The effects of global climate change are expected to grow substantially over time.