That was one toxic sermon.

Carbon-monoxide fumes in a French church during Mass on Christmas Eve sickened 72 people and sent 21 of them to the hospital — including two in serious condition, according to a report.

Worshipers at the Saint Eloi Church in Carlepont, about 75 miles north of Paris, began feeling woozy during Tuesday’s service, prompting officials to evacuate the structure, the Daily Mail reported Wednesday.

Nineteen people were taken to local hospitals, while the two with more severe symptoms were taken to special care centers, where one was placed in a hyperbaric oxygen chamber, the Mail said.

Local officials have not determined the source of the toxic fumes but ordered the church closed until an investigation into the mass poisoning is completed.

Carbon monoxide is an odorless, invisible gas that can cause headaches, fatigue and nausea at levels above 70 parts per billion and can prove fatal at levels above 150 to 200 parts per million, according to the US Consumer Product Safety Commission.

Nicolas Mougin, a French emergency official, said CO levels inside the Carlepont church were as high as 350 parts per million.