Phoenix, AZ – Swimming holes are popular during the hot-summer months in Arizona, but some local doctors are warning people to be careful before heading out to a lake or river.

Doctor Robert Baron of Good Samaritan Medical Center says carbon monoxide poisoning at lakes and rivers is a possible reality.

“Since 1990, we have kept records and have now had over 800 cases of people poisoned by carbon monoxide just in the United States,” Baron told KTAR.

He says excessive exposure to boat engines on lakes and rivers can cause carbon monoxide poisoning.

“Boat engines do not have catalytic converters on them,” Baron said. “Because of that, they produce 188 times more carbon monoxide than the same engine if it would have been put into your car.”

Baron says people should try to limit their time near boat engines while at the lake or river.