3 reports in this theme, newest first.
The world adds 1.43 billion people by 2050, 69% of them in cities and 57% in sub-Saharan Africa. People living with 30 or more days a year above a 35 °C heat index more than double, from 1.9 to 4.1 billion. Warming drives 1.74 billion of that rise and population growth 0.44 billion, and growth lands disproportionately in hot places. City dwellers in 100-year flood zones, 507 million today, are heading for 630–690 million.
Pollution and people concentrate in the same places: the average person breathes air 1.53 times dirtier than the average inhabited square kilometre. But most of a city's pollution is not its own — in the median city the background 165 km away is 79% as polluted as the centre, and the wind carries city smoke downwind. Exposure is most unequal by income: 41.7 µg/m³ for lower-middle-income countries, 3.2 times the high-income level — so clean air needs regional, not just urban, action.
The poorest half of humanity breathes 66% of the world's fine-particle exposure above the WHO guideline. Exposure peaks not in the poorest countries but in lower-middle-income ones, where industry, dense cities and solid-fuel cooking coincide: their population-weighted average is 42.5 µg/m³, against 12.5 in high-income countries. Since 2010 three in four high-income countries have cleaned their air, against fewer than half of lower-middle-income ones.