A carbon monoxide leak that sickened 18 people and forced the evacuation of a beachfront Ocean City resort Friday was caused by a ventilation issue with the hotel’s boiler, officials said.

The leak prompted a large emergency response and temporarily halted operations at the Ashore Resort & Beach Club, a 250-room resort on Coastal Highway.

“Following a thorough inspection by the fire department and the city’s building inspector, the cause of the carbon monoxide leak was determined to be a ventilation issue with the boiler,” said Kane Kennedy, general manager of the resort. “The boiler has been shut down, and repairs are being made.”

Shortly after 11 a.m., the Ocean City 911 Center dispatched fire and EMS units to the hotel in response to a carbon monoxide alarm activation.

Eighteen people were assessed on the scene. Four adults were transported to Atlantic General Hospital in stable condition. All were employees working in a first-floor office area adjacent to the building’s boiler room.

Fire department personnel cleared the scene at 1:21 p.m. Officials with the Ocean City Fire Marshal’s Office and Town of Ocean City Building Inspectors and representatives of the gas company remained on-site to conduct an investigation.

“At this time, it has been determined that the source of the carbon monoxide was a boiler located on the ground floor of the building,” Fire Marshal Joe Sexauer said. “In order for this boiler to be turned back on, town officials will require necessary repairs, followed by a third-party mechanical inspection and a gas company inspection to ensure the system is safe to operate.”

Due to the number of patients requiring simultaneous evaluation, the incident commander declared what Maryland EMS protocols define as a mass casualty incident. This designation is not related to the severity of injuries or presence of fatalities, but it signals that the number of patients temporarily exceeds the capacity of initial EMS resources on scene.

“Unfortunately, some scanner platforms and social media pages reported the incident as a ‘mass casualty’ without context, which led to unnecessary fear in the community,” said Ryan Whittington, community & department engagement officer for the Ocean City Fire Department. “To be clear, there were zero fatalities. In Maryland, the term ‘mass casualty incident’ is used purely to describe a situation where multiple patients require care beyond the capabilities of the initial EMS response. It does not imply deaths or critical injuries.”

This incident highlights the importance of carbon monoxide alarms.

The Ocean City Fire Department reminds all residents, businesses and property managers to ensure that carbon monoxide alarms are properly installed, tested regularly and maintained in accordance with manufacturer guidelines and local codes.

Guests and staff were allowed back into the hotel around 1:30 p.m. Friday, after officials deemed the building safe.

“Normal operations have resumed,” Kennedy said.

Before being bought by Certares Real Estate Management, Monomoy Property Ventures and Highlands Investments in 2022, the hotel was known as Ocean City Fontainebleau Resort.

Baltimore Banner reporters Darreonna Davis and Nori Leybengrub contributed to this story.