BALTIMORE, SEPT. 3 -- A card enclosed annually in utility bills to help customers identify the smell of a natural gas caused trouble instead of preventing it when the bill's potent odor was mistaken by hundreds of residents as a gas leak, officials said today.

"It's a safety measure. We have been sending them for many, many years once a year to meet a public service commission requirements," said Baltimore Gas and Electric Co. spokesman John Metzger. "This time it arrived at people's homes and I don't know if the odor was stronger, but there were calls to the fire department and hundreds of calls here by people saying they had a gas leak."

Capt. Patrick Flynn, Baltimore Fire Department spokesman, said the problem came to his attention Wednesday afternoon when a woman called the fire department reporting a gas leak.

"She said she was getting sick from the smell. We sent out four engines, two trucks, a battalion chief and an ambulance -- 27 men," Flynn said.

But after searching for the leak in all the logical places -- the stove, water heater and furnace -- one firefighter saw the card with a small red flame on the table. "He picked it up and smelled it. He said you could smell it right through the envelope," the spokesman said.

That call was followed by hundreds of others, but by then officials were aware that the gas smell was coming off the three-inch cardboard slip in this month's utility bill.

"My nose is perhaps not as sensitive as some people's," Metzger said. "Maybe it is slightly stronger than usual but I understand that there was no more odor applied to the card than usual."

Metzger said the card is mailed every year before the winter season when people winterize their homes and apartments.

BG&E is considering what to do with the cards, which must be included to comply with public service regulations. Until the problem is solved, the utility is temporarily discontinuing the service.

Natural gas is odorless. The smell on the card and the odor perceived during a natural gas leak is methyl mercaptan. It is added to natural gas in small quantities to alert people to gas leaks.