US embassies’ tweets showing real-time air quality data resulted in lower levels of air pollution in cities around the world. In 2008, the US embassy in Beijing installed an air quality monitor and began tweeting hourly readings. By 2020, more than 50 US embassies in 37 other countries had followed, creating a large data set that researchers could use to assess the impact of disseminating real-time pollution data. Akshaya Jha at Carnegie Mellon University in Pittsburgh, Pennsylvania, and his colleagues analysed satellite data on air pollution in 466 cities in 136 low and middle-income countries, including 50 cities where US embassies installed monitors. They focused on pollution particles with a diameter of 2.5 microns or less, known as PM2.5. These particles are particularly harmful because they penetrate the lung barrier and enter the bloodstream, contributing to respiratory and cardiovascular conditions such as heart disease and lung cancer. The study found that cities where US embassies tweeted air contamination data saw the levels of PM 2.5 drop by an average of 2 to 4 micrograms per cubic metre compared with those that didn’t. (https://www.newscientist.com/article/2343859-pollution-tweets-from-us-embassies-improved-air-quality-in-cities/) In 2016, it was estimated that outdoor air pollution caused 4.2 million premature deaths worldwide per year, but only 32 per cent countries in the study sample had any form of air quality monitoring. Fewer countries still made that data public.