SAN DIEGO -- Carbon monoxide is an invisible and odorless gas. When it builds up, it can kill or cripple a victim who is unknowingly breathing it, 10News reported.

CARBON MONOXIDE What Is It? Protect Your Home Symptoms Of CO Poisoning CO Detectors Have A CO Detector?

Many safety experts recommend homes be equipped with carbon monoxide alarms. 10News put some of the most popular alarms to the test and found they do make noise when they read high enough levels. But some safety experts said they would like to see the alarms go off much earlier.

In the investigation, two alarms by Kidde (the Basic and Nighthawk) and one by First Alert (a smoke alarm/carbon monoxide detector combination) sounded when the carbon monoxide level in the test room reached 306 parts per million. It took about 30 minutes of stoking the test fire -- burning charcoal in a Hibachi -- to reach the potent level and set off the alarms.

The response fell within standards set by Underwriters Laboratories, but the firefighters who monitored the test said that's not good enough.

"These are not satisfactory," said firefighter Eric Thomas. "The alarms are not loud enough and don't sound often enough. That was a hazardous environment and there was no indicator in the first few minutes that people needed to get out. That's the most important thing."

However, the alarm makers disagreed.

BRK Brands, which makes First Alert, said residential carbon monoxide alarms are very different from commercial multigas sensing meters, which typically sell for $2,000 and more.

Spokeswoman Deborah Hanson said, "Commercial detectors can be set to detect and sound at a variety of exposure levels. Residential carbon monoxide alarms have one fixed setting."

Kidde spokeswoman Laurie Bowser-Sever said, "Kidde's residential carbon monoxide alarms are designed to alarm before someone is expected to feel the effects of carbon monoxide poisoning."

She said alarms save lives.

The UL's chief toxicology advisor agreed.

Dr. Jerrold Leikin of Evanston Northwestern Healthcare said a survey of 4,500 carbon monoxide emergencies (between the years 1994 and 1998) showed their were no deaths when a working carbon monoxide detector was in the house. He said the evidence was so overwhelming that in Chicago there is an ordinance that requires carbon monoxide detectors in many homes.

New York is the most recent city to adopt a carbon monoxide alarm ordinance.

Leikin added that the concentration of carbon monoxide would have subjected many to about one-third the poison needed to kill during a short-term exposure.

Dr. Jake Jacoby, the head of Hypobaric Medicine at the UCSD Medical Center, said differently.

Jacoby told 10News that if exposed over the course of several hours, a quarter of the exposure -- 75 ppm -- could be very harmful. Firefighter Eric Thomas said his professional monitor is set far lower than residential alarms because of the potential harm of breathing carbon monoxide while working or exerting energy.

Jacoby recommended that families install carbon monoxide alarms in their homes.

Experts differ on where to place them.

Jacoby said sleeping and living areas are good spots for carbon monoxide monitors. Thomas recommended putting alarms in areas near potential carbon monoxide sources: kitchens, near furnaces, water heaters and other fuel-powered appliances (outside a 10-foot radius to prevent nuisance alarms). Place alarms in stairwells and hallways, that way alarms can respond to higher gas levels before they reach the bedroom or lounging areas of the house, where families could be caught unaware, with little or no time to respond.

Here are other recommendations:

Before turning on the furnace for the first time this year, call San Diego Gas and Electric for a furnace checkup. Make sure furnaces, water heaters and chimneys are working right.
Clean chimneys on a regular basis, so they don't back up and poison the family.
Never use a gas oven to heat the house. It can produce deadly carbon monoxide.
Absolutely never burn a BBQ or other fuel-powered heater or appliance indoors.
Kidde recommends changing out alarms every five to seven years because of changing technology, and replacing batteries every year. Underwriters Laboratories recommends testing carbon monoxide alarms at least once a month.
If your carbon monoxide alarm does go off, clear the house and call the fire department or 911 for help and instructions.