Methane (CH4) is a greenhouse gas that has a residence time in the atmosphere of 9-15 years. Methane is more than twenty times more effective in trapping heat in the atmosphere than carbon dioxide. Although some natural sources emit methane to the atmosphere, the gas is also emitted from human sources including landfills, natural gas and petroleum systems, agricultural lands, coal mines, automobile combustion, wastewater treatment plants, and other industrial process. The concentration of methane in the atmosphere has increased from about 700 to 1700 pbb since the onset of the Industrial Revolution. Information source: U.S. EPA.
Global Greenhouse Gas Levels
The major greenhouse gases are globally mixed and their current levels have greatly exceeded their pre-industrial levels. The current levels of greenhouse gases are all considered unhealthy.
See evidence of just how drastically their levels have increased in the ice core record.
There are no known direct health impacts associated with atmospheric methane. However, because methane is a potent greenhouse gas, it can impact health indirectly with rising concentrations contributing to global climatic change. Information source: U.S. EPA.