Fire crews were called out to the 100 block of West Airy Street in Norristown around 11:00 p.m. Saturday.
Firefighters found elevated readings in five homes.
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Officials say the source of the leak was a generator running in the basement of one of the homes.
"Power to the home was shut off for non-payment by the utility company on the 21st of this month and sometime between then they went out and purchased a new generator and they were running that to power the home," Norristown Fire Chief Tom O'Donnell said.
Eight people were taken to the hospital, including a paramedic.
Two are in critical condition.
Crews also rescued two dogs who needed to be given oxygen.