Blue hydrogen is hydrogen produced from natural gas using a process called steam methane reforming, where natural gas is mixed with very hot steam and a catalyst. A chemical reaction occurs creating hydrogen and carbon monoxide. Water is added to that mixture, turning the carbon monoxide into carbon dioxide and more hydrogen. If the carbon dioxide emissions are then captured and stored underground, the process is considered carbon-neutral, and the resulting hydrogen is called "blue hydrogen". The carbon capture and storage (CCS) technology is used to trap and store the carbon dioxide produced during the process, which makes it a low-carbon energy carrier. However, the efficiency of the CCS system is around 60-65%, which means that 30-35% of carbon dioxide will still be emitted.