Twenty-one patients were treated for carbon monoxide poisoning on a Lake Powell houseboat early Wednesday morning, Glen Canyon National Recreation Area officials say.

Glen Canyon National Recreation Area Rangers identified 21 patients on a privately owned houseboat in Face Canyon.

Five people — three adults and two children — were flown to a hospital in St. George for additional treatment. The rest of the patients were treated and released at the scene.

The National Park Service reports that this is the third carbon monoxide-related incident at Lake Powell in July so far. The agency is currently investigating the source of the carbon monoxide.

Carbon monoxide is a colorless, odorless, toxic gas that impairs the ability of blood to carry oxygen throughout the body, according to the U.S. Centers for Disease Control and Prevention. Vehicle exhaust is a common source of carbon monoxide.

Exposure to the gas can cause headaches, nausea and exhaustion. More acute exposure can lead to brain and heart damage.