To limit the probability that inadequate airtightness of manhole covers will lead to the entry of sewer gases into occupied space, which could lead to negative effects on indoor air quality, which could lead to harm to persons.
To limit the probability that inadequate venting will lead to a pressure buildup of sewer gases in manholes, which could lead to the entry of sewer gases into occupied space, which could lead to negative effects on indoor air quality, which could lead to harm to persons.