Baltimore firefighters were investigating the cause of a carbon monoxide leak Friday that prompted the evacuation of 48 people — 36 children and 12 adults — at the Pleasant View Gardens Child Care Center.

Fire union officials initially described it as a "mass casualty carbon monoxide incident" and said the victims were being hospitalized, but reports from the scene indicated that those affected were being evaluated outside the building in the 1100 block of E. Fayette St. and no one appeared to be showing symptoms of carbon monoxide poisoning.

Fire spokesman Chief Kevin Cartwright said a carbon monoxide detector went off about 11 a.m. at the child care center, an administrator called 911 and the building was evacuated. Hazmat units initially registered carbon monoxide readings of 68 to 70 parts per million, and the building was being ventilated.

According to the National Institutes of Health, the color-less, odor-less gas can cause sudden illness and death. It is found in combustion fumes, such as those made by heating systems, and can build up in places that don't have a good flow of fresh air. The most common symptoms of carbon monoxide poisoning are headache, dizziness and nausea.