Gas fumes, possibly from a cleaning product in use for maintenance, sent three people to Sinai Hospital in Baltimore and forced the evacuation of 200 people from the Ellicott City District Court building Thursday afternoon.

The three people sent to Sinai are in serious condition for exposure to what investigators believe was chlorine powder being used by a contract maintenance crew working on the ventilation system. A fourth visitor to the building was treated at the scene and a county police officer was treated and released. Five county police officers waiting for court appearances retrieved gas masks from their vehicles and helped evacuate the building as Howard County fire hazardous materials teams responded for what is thought to be an accident. One officer was treated and released.

Fire Department spokeswoman Julia Lynch said all three people hospitalized were conscious and breathing but had symptoms that required further examination and treatment. All were visitors to the building.

Officials said 70 emergency vehicles responded to the scene at about 2 p.m., but said court should be open Friday.

larry.carson@baltsun.com