CHICAGO (CBS) — Paramedics responded to a family complaining they felt ill and discovered an increased level of carbon monoxide late Saturday at a home in west suburban Naperville.

About 9 p.m., a person called 911 to report multiple family members feeling sick at the home in the 1000 block of Bradford Drive, according to a statement from the Naperville Fire Department. They were advised to wait outside the building as firefighters and paramedics were dispatched.

Six minutes later, firefighters detected an increase in carbon monoxide levels, authorities said. The incident was elevated to an ambulance box alarm, bringing four additional ambulances and two commanders to the home.

Four people were treated with “advanced life support protocols” and taken to Edward Hospital in Naperville, fire officials said.

An investigation revealed there was a problem with the building’s water heater, authorities said. Adjoining units in the building were checked for carbon monoxide poisoning and had slightly elevated readings. All carbon monoxide levels were quickly brought within normal standards through ventilation.

The unit from which the call was made was not equipped with a working carbon monoxide detector. A representative from NICOR was requested to the home for an evaluation.

(Source: Sun-Times Media Wire © Chicago Sun-Times 2016. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.)