Wayland, MA- A Wayland family returned to their Bayfield Road home in October to an unwelcome surprise: Their newly installed carbon monoxide detector was sounding its alarm. Firefighters who rushed to the scene found that the air in the home contained 205 parts per million carbon monoxide, a potentially deadly level.

The family suffered no injury or illness from the gas, which firefighters traced to an improperly installed heating unit. But without the detector, they might not have been so lucky.

“If you stay in that long enough, you’re not coming down to breakfast,” said Fire Chief Robert F. Loomer.

Nine months after a state law required carbon monoxide detectors to be installed in most residences, such calls are becoming more common, according to fire chiefs across Boston’s western suburbs. They said they believe the detectors are improving people’s health — and sometimes saving their lives.

From April to December 2006, fire departments statewide fielded 4,737 carbon monoxide calls, according to preliminary state figures. That was a 44 percent increase over the 3,281 calls received in the same period a year before. The state is still tabulating 2006 calls, and the increase is likely to be even larger than 44 percent, said state fire marshal’s spokeswoman Jennifer Mieth.

“We think this has been an underreported problem and an undetected problem for a very long time,” she said.

Nicole’s Law, passed in 2005, was named for Nicole Garofalo, a 7-year-old who died after a blocked furnace vent filled her Plymouth home with carbon monoxide. It mandates detectors in all residences with potential sources of carbon monoxide.

The state Board of Fire Prevention Regulations gave homeowners and landlords until March 31, 2006, to install the devices, and some large complexes were given until the first of this year. “Transient” residential buildings, such as hotels, hospitals, and jails, have until Jan. 1, 2008, to install detectors.

In Framingham, fire officials said they didn’t have monthly figures, but calls for 2006 rose to 113 from 66 the year before. Franklin reported 47 calls in 2006, up from 32.

The Natick department, which was able to provide monthly call figures, said it responded to 26 calls from March 31, 2006, to the end of the year, compared with 17 during the same period a year before.

Chiefs of several other area departments said they also had noticed an uptick but could not provide numbers.

In Newton, Fire Chief Joe LaCroix said he hasn’t noticed an increase yet, but he expects that to change as temperatures dip and the state goes through its first full heating season under the new regulations. Faulty heating systems are a prime cause of carbon monoxide buildup.

The colorless, odorless gas, a byproduct of combustion, is produced in higher amounts when fuel isn’t fully burned, such as in a malfunctioning gas stove, and can build up in homes as a result of blocked chimneys, and cars left running in attached garages.

According to the US Consumer Product Safety Commission, people begin to show flu-like symptoms as levels increase past 70 parts per million. Prolonged exposure to levels above 150 to 200 parts per million can cause disorientation, unconsciousness, and death.

Many of the calls received by fire departments turn out not to be critical situations. For example, a person might warm up a car in the garage in the morning, then return home in the afternoon to a CO alarm. Calls like those will likely taper off over time as people get used to the detectors and adopt safer practices, several chiefs said.

“I think, of course, there’s a learning curve,” said Wrentham Fire Chief Robert Morrill. “We had this same effect when smoke detectors first came in.”

Morrill didn’t have specific figures but said his department had seen an increase in calls. He said the calls represent a small percentage of his department’s activity, and he isn’t overburdened by the increase. On the contrary, he’s glad to see that people are installing the detectors and contacting authorities when they sound.

Plainville Fire Chief Ted Joubert said the detectors are “absolutely, without question” saving lives. Fire officials will ensure that the detectors are installed during presale inspections of homes, but Joubert said residents should install them anyway for their own safety.

“Anybody who doesn’t have one, I’d have to urge them to rethink that position,” Joubert said. “They’re truly as valuable as a smoke detector.”