This is a short advice note from the Air Quality Expert Group to the Department for Environment, Food and Rural Affairs; Scottish Government; Welsh Government; and Department of Agriculture, Environment and Rural Affairs in Northern Ireland, on the scientific and technical issues associated with the combustion of hydrogen gas that may be relevant to air quality. The information contained within this report represents a review of the understanding and evidence available at the time of writing. Expanding the use of hydrogen as a low carbon fuel has some potential implications for air quality if it is used as a combustion fuel. Using hydrogen to power fuel cells that directly generate electricity does not create any air pollution at point-of-use. When hydrogen is burned in engines, boilers and furnaces the very high temperature of the flames can split apart nitrogen (N2) in the air leading to the formation of nitrogen oxides (NOx), an important class of air pollutant. Hydrogen burns with a hotter flame than most fossil fuels and has the potential to emit more NOx per unit of heat that is generated. Burning hydrogen gas instead of hydrocarbon fuels (e.g. gasoline, diesel, or bioderived fuels such as ethanol) can bring benefits for air quality, producing lower emissions of particulate matter and eliminating carbon monoxide. Reducing emissions of NOx from the combustion of hydrogen can be achieved in many situations using existing exhaust aftertreatment technologies, by lowering the temperature at which the hydrogen is burned and by optimising the ratio of fuel to air. This sometimes leads to additional cost and/or a reduction in energy efficiency. There are few commercially available examples of engines or boilers that have been specifically designed to burn hydrogen and real-world data on emissions performance is very limited. However, if effective NOx emission controls (both technical and regulatory) are in place then hydrogen has the potential to be a substantially cleaner fuel from an air quality perspective than the fossil fuels it may replace.