you'd be surprised how many places there is somewhat dangerous levels of methane in the atmosphere tho. more common where there is earthquakes but a lot more of a danger to people than recognised.

it's mostly not a problem for me because i spend most of my time on the lower storey with a big space for methane to float upwards, but carbon dioxide/monoxide is heavier than air, and what i think was happening was the levels of CO and CO2 were building up in my space with all the windows closed.