Hydrogen fuel cell vehicles (FCVs) are another alternative to internal combustion engine vehicles, but are less than half as efficient as electric vehicles (EVs). Most hydrogen is extracted from natural gas, which, when processed, emits carbon dioxide and methane, according to a 2021 study. Thus, it creates far more greenhouse gas emissions than electricity. FCVs have some engineering challenges to overcome before they will be widely available in the light duty passenger vehicle market; these include vehicle cost, hydrogen cost, hydrogen storage and delivery, and competition with other technologies such as EVs. The electrical grid already exists for plug-in vehicles, and current trends show that plug-in vehicles are quickly becoming the dominant alternative to internal combustion engine vehicles. As of August 2021, 11,000 fuel cell vehicles have been sold in the U.S., compared to more than two million plug-in EVs.