Yes, crawlspaces and basements where the gas can build up underneath a home and seep in. If the house does not have a crawlspace or basement I would doubt there would be much concern. Not every area has the necessity or elements needed to produce the gas either so that is why I said if the house had a basement or underground lower level that would be utilized quite a bit it would be important to check out what the composition of the earth beneath the home is compromised of.