Climate change and environmental epidemiology

In a broader context of epidemiologic transition and economic development, the changing age and geographic distribution of the global population will continue transforming environmental health-research priorities. Average lifespan has gone up across the globe, including important gains in low-income countries where life expectancy has improved from 53 years in 1990 to 62 years in 2012 (WHO 2014). Adults aged 60 and older comprised 8.0 per cent of the global population in 1940, which grew to 12 per cent by 2013 and is projected to be 21 per cent in 2050, according to the UN Department of Economic and Social Affairs (UN DESA), 2013. This significant shift in the global age distribution is due in part to public health interventions, but also presents new health challenges.

There will also be notable shifts in the geographic distribution of the global population. Most of the population growth between now and 2050 is projected to occur in just nine countries with high fertility or already large populations. More than half of the expected growth will be in Africa alone. The proportion of the population living in Africa will increase from 16 per cent to 25 per cent between now and 2050, while the proportion in Europe will shrink from 10 per cent to 7.0 per cent, projects UN DESA, 2015. Population growth is projected to remain especially high in the 48 least developed countries, adding to challenges in meeting sustainable development goals. There is currently very little data on environmental exposures and their health effects in many countries with the largest population growth. Effects of environmental exposures are likely to differ from those observed in high-income countries, where most environmental health research has been conducted, due to differences in infectious disease burden, access to health services, and material deprivation.

Large-scale migration will add further complexity to environmental epidemiology, presenting challenges for follow-up of study participants and environmental exposure assessment. Individuals living in areas with similar levels of environmental exposures may have highly variable cumulative exposure on the basis of their migration history. For migrants from poorly to better regulated societies, adult health may be influenced by high levels of environmental exposures in early life --exposures which may be particularly difficult to reconstruct. Migration may also present opportunities for using natural experiments to understand how environment shapes health.

The global population continues to shift from rural to urban areas. In 2014, 54 per cent of the population resided in urban areas, which is projected to be 66 per cent by 2050, according to UN DESA, 2014. Nearly 90 per cent of the projected increase in the world's urban population will be concentrated in Asia and Africa, with India, China, and Nigeria accounting for a large share of this growth. Urbanisation profoundly shapes --both positively and negatively -- environmental exposures (e.g. air pollution, noise, green space) and behaviours (e.g. physical activity, food consumption) and thereby disease risks.

Scientists have warned that in the next few decades, key agricultural areas in the Indus and Ganges river basins will be affected, which will result in reduced crop yields and famine. "Climate change is not an abstract concept, it is impacting huge numbers of vulnerable people," MIT Professor Elfatih Eltahir told the Thomson Reuters Foundation. "Business as usual runs the risk of having extremely lethal heat waves." As of now, only 2.0 per cent of India's population is exposed to extreme weather but by 2100 about 70 per cent of the people will suffer. Media report that around 3,500 people were killed by heat waves in 2015. Projections show the Gulf region will be the world's hottest region by 2100 as a result of climate change.

Climate change and emerging environmental risks will define much of the future context for environmental epidemiology. Mean surface temperature is expected to increase by 0.3 to 4.8 °C by 2100, leading to direct impacts on health from heat stress and flooding, as well as indirect health impacts mediated through infectious diseases, air quality, and food security.

Recent reports have elaborated the multiple potential health impacts of climate change. The Intergovernmental Panel on Climate Change anticipates an increase in both the frequency and intensity of heat waves under all climate scenarios. The combined effect of global warming and demographic change will expose an increasing number of vulnerable older adults to heat stress. Health impacts of extreme weather events such as storms and floods are likely to increase this century if no adaptation measures are taken. It is also anticipated that climate change increases the risk of intense droughts in some regions, affecting agricultural output and, subsequently, increasing food insecurity and malnutrition. Droughts also increase risks of water-related disease (e.g. E coli, cholera), vector-borne disease (e.g. dengue, West Nile Virus), airborne and dust-related disease and mental illness. Climate change may be an important factor in the dynamics of vector-borne disease transmission, including malaria, dengue, and Lyme's disease. Alongside shifts in land use, climate change appears to be altering the geographic range of vectors that transmit pathogens to humans.

Chemical exposures will remain an important environmental health concern. Chemical production in 2000 was 1,000 times higher than in 1930.

Technology with applications to environmental exposure and health outcome assessment is evolving rapidly. Technology will generate new opportunities, particularly in regard to population datasets, e- and m(obile)-health, personal and remote sensor technology. Expanding data availability will allow prediction of diverse population exposures and create new opportunities for exploring novel exposures that were previously difficult to quantify. Importantly, geo-referenced data are becoming more widely available in low- and middle-income countries, reducing barriers for conducting environmental epidemiology in these countries. Such data include those collected through remote sensing, sensor networks, smartphones, as well as the "internet of things" (i.e., everyday objects with network connectivity). Remote sensing has been used to estimate environmental exposures including air pollution, green space, and temperature. Opportunities for satellite-based exposure assessment will continue to expand with increasing number of satellites and improved resolution of detection.

A new exposure pathway -- visual exposure -- will be easily investigated using miniaturised cameras or virtual reality to understand how people internalise and interact with their environment. For example, such technology will allow advances beyond simple proximity to green space to determine whether individuals are visually exposed to green space and which activities they engage in using that green space. Social media data will increasingly play a role in assessing behaviours, exposures and outcomes. Such approaches have already been used to identify symptoms, behavioural risk factors, and population mobility patterns.

 The writer is former Head, Department of Medical Sociology,

Institute of Epidemiology, Disease Control & Research (IEDCR),

Dhaka

[email protected]