Gray hydrogen is a type of hydrogen produced from natural gas through a process called steam methane reforming. In this process, natural gas reacts with steam to produce hydrogen and carbon dioxide. The carbon dioxide is released into the atmosphere, contributing to greenhouse gas emissions. This method is the most common way to produce hydrogen today, but it is not environmentally friendly due to its carbon footprint. In contrast, green hydrogen is produced using renewable energy sources, such as solar or wind power, which do not emit carbon dioxide during production.