New data in a report from the UN’s global health body “is enough to make all of us extremely concerned,” Maria Neira, the head of the WHO’s department of public health and environment, told reporters.

The problem is most acute in cities, but even in rural area, the air is not as clean as many think, WHO experts said.

The data focuses on dangerous particulate matter with a diameter of less than 2.5 micrometres, or PM2.5. PM2.5 includes toxins like sulfate and black carbon, which can penetrate deep into the lungs or cardiovascular system.

Air with more than 10 micrograms per cubic metre of PM2.5 on an annual average basis is considered substandard. In some regions satellite data has been complemented by ground-level PM2.5 measurements, but in much of the developing world ground readings remain unavailable, forcing the WHO to rely on cruder estimates.

Despite these data gaps, Neira said the UN agency now had more information than ever about pollutants in the planet’s air. Using both satellite and ground measurements “is a big step forward towards even more confident estimates of the huge global burden” of dirty air, she added.

Data is more solid for outdoor pollution, which is blamed for more than three million fatalities annually. But indoor pollution can be equally as harmful, especially in poorer developing world homes where cooking often involves burning charcoal.

Southeast Asia and the Western Pacific region – including China, Malaysia and Vietnam – are the hardest hit, the data showed. Using a different data set, the WHO reported in May that 80 per cent of the world’s city dwellers breathe poor quality air, a figure that rose to 98 per cent in poorer countries.