Authorities are still investigating what caused seven people to get sick from carbon monoxide poisoning in northern Minnesota last weekend.

All were hospitalized after firefighters were called to a home in Virginia, Minn., last Sunday. Five first responders also had to be evaluated for carbon monoxide exposure.

Authorities said Thursday that all but one person has since been released from the hospital. While the cause of the poisoning is under investigation, Virginia Fire Marshal Chris Clark said the home's boiler furnace had some issues.

Carbon monoxide poisoning is a hidden danger because the toxic gas is odorless and invisible. Symptoms include headaches, dizziness, nausea, vomiting and chest pain.

In 2016, five people died in Minnesota from carbon monoxide poisoning — down from a recent record high of 18 deaths in 2013, according to state health data.

In winter months, sources of carbon monoxide include fumes from heaters or fireplaces. Officials recommend checking carbon monoxide detectors and clearing space around appliances.

Starting next summer, Sophia's Law, named for 7-year-old Sophia Baechler of Edina, who died in 2015 on Lake Minnetonka when carbon monoxide leaked from an exhaust pipe on a boat, will require an estimated 8,000 boats with "enclosed accommodation areas" to have a marine-certified carbon monoxide detector. The law is the most stringent of its kind in the U.S.

An estimated 45,000 boats with "enclosed occupancy spaces" will also need warning stickers about CO poisoning.