Carbon Dioxide is a naturally occurring colourless and odourless gas. Mainly associated with landfill sites, carbon dioxide is an asphyxiant. At volumes as low as 3% carbon dioxide can cause headaches and nausea, symptoms worsen at the volume increases. At around 20% loss of consciousness and suffocation could occur.
Carbon dioxide protective measures are the same as those for methane, it is very common for both types of ground gases to be present in the same location.