Today, most hydrogen is used as a feedstock in petroleum refining and fertilizer manufacture. These industries have a number of harmful emissions. Many of the envisioned new uses of hydrogen for transportation involve consuming hydrogen in fuel cells to produce electricity, emitting only pure water as the effluent.
In hydrogen applications that involve combustion in air, most, but not all, criteria air pollutants are eliminated. When hydrogen is combusted in air, oxides of nitrogen (NOx) can form in addition to water vapor. Because hydrogen burns hotter than methane and other hydrocarbons, greater attention is required to limit NOx formation during combustion than with most other fuels. NOx (principally nitrogen oxide [NO] and nitrogen dioxide [NO2]) affect tropospheric ozone and contribute to acid rain and photochemical smog, which has known adverse health effects including asthma.
Use of hydrogen combustion is likely in the future in a range of new applications in order to reduce carbonize emissions. These applications may include industrial process heat, power generation in peaker plants (where fuel cells are deemed uneconomic due to low capacity factors), and possibly in transportation applications, such as ocean shipping, trains, and aviation.
 Criteria air pollutants are particulate matter, photochemical oxidants (including ozone), carbon monoxide, sulfur oxides, nitrogen oxides, and lead. EPA named these pollutants “criteria air pollutants” because it sets national ambient air quality standards for them based on criteria that capture the latest scientific information on their impact of human health and welfare.