3.2.4.6 - Environmental change and population futures

Ozone change, ultraviolet exposure and health

Stratospheric ozone absorbs much incoming ultraviolet radiation. Depletion of this protective layer can increase the ultraviolet radiation reaching people; it differs from harmful ozone pollution near the ground.

Human-made ozone-depleting substances release chlorine or bromine in the stratosphere. These participate in reactions that destroy ozone. More ultraviolet exposure can damage skin-cell DNA and increase skin-cancer risk. It can also damage the eye's lens, contributing to cataracts, where clouding impairs vision. WHO's ultraviolet evidence identifies both skin and eye effects; exposure duration, outdoor work, protection and individual susceptibility mediate risk.

A depleted layer does not mean everyone develops a disease. Latitude, season, cloud conditions and behaviour affect actual exposure, while diagnosis and access to care influence recorded illness. A comparison of country cancer totals alone cannot isolate ozone as the cause: population size, age and previous exposure differ.

The Montreal Protocol controls ozone-depleting substances. This demonstrates that an environmental pressure can be changed through international action, although atmospheric persistence makes recovery gradual. Protection from ultraviolet exposure and equitable access to detection and care address nearer-term risks. Keep the ozone pathway separate from greenhouse warming; the processes can interact but are not interchangeable explanations.

Warming, thermal stress and vector disease

Climate change can affect health directly through heat and indirectly by changing disease ecology, but vulnerability determines who is harmed.

High temperatures make it harder for the body to lose heat. Dehydration and heat stress can increase illness and mortality, especially among older people, people with existing illness and outdoor workers. Hot nights reduce recovery. Urban surfaces and limited shade can intensify exposure; poor housing or inability to afford cooling can convert a similar hazard into greater risk. Cold-related illness may fall in some settings, but this does not cancel every heat effect or establish one global net outcome.

Warming can alter mosquito development, parasite development and the length of suitable transmission seasons. Where temperatures were previously limiting, suitable conditions may extend to higher elevations or latitudes. However, excessive heat or drought can reduce vector survival or breeding sites. Rainfall, drainage, vector species, host contact and disease-control capacity remain important: a temperature projection alone is not a forecast of malaria cases.

The WHO climate and health account describes multiple interacting pathways. A useful causal chain specifies hazard, exposure, vulnerability and response. Heat warnings, accessible cool spaces and well-designed housing can reduce thermal risk; vector surveillance, protected housing and effective control can reduce transmission. Evaluate the people reached, response capacity and changing conditions, rather than assuming adaptation eliminates risk.

Agricultural change and nutritional standards

Environmental change affects nutrition through food production and access, so a harvest change does not translate mechanically into a health outcome.

Heat can shorten crop development or damage flowering; drought reduces soil water, and flooding can waterlog roots or disrupt transport. Shifts in season length may benefit some cooler regions or allow different crops, while pests and extreme events can offset those gains. Irrigation, breeding, soil protection and altered planting dates can adapt production, with costs and water limits.

Lower or less reliable supply can raise prices and reduce the quality and variety of diets. A household may maintain calories with cheaper foods while losing protein or micronutrients. Illness can then reduce work and income, further limiting food access: this is a reinforcing feedback. Trade and storage can buffer local shortages, but dependence on one supplier or transport route can transmit distant shocks.