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Hydrogen holds promise as a clean, low-carbon fuel (see, for example, go.nature.com/3arm2nq). But under current plans — including those in the United Kingdom’s sixth carbon budget (see go.nature.com/3hjkgvp) — the gas would be mostly burnt in engines and boilers rather than being used in fuel cells. The burning of hydrogen generates toxic nitrogen oxides (NOx) as well as steam (Y. B. Zel’dovich Acta Physicochimica URSS11, 577–628; 1946). This pollution could disproportionately impact the urban poor.