CHICAGO, Il. (WLNS) – Four beeps mean it’s time to get outside immediately at UL Solutions near Chicago.

“That is what’s called a four-temporal pattern. It’s what we refer to it as,” said David Mills, Principal Engineer at UL Solutions.

Mills runs hundreds of tests on alarms that are critical to detecting carbon monoxide gas before it kills.

“You can’t see it. You can’t smell it. You can’t taste it,” said Mills.

Those four beeps mean the clock is ticking.

The higher the concentration of carbon monoxide, the faster the alarm goes off.

“The effects of CO poisoning would be happening faster at the higher concentration versus at the lower concentration,” said Mills.

Without an alarm, Ashley Wilson had no idea the flu-like symptoms she was experiencing were being caused by carbon monoxide seeping into her Wisconsin apartment.

The gas was slowly poisoning her and her family.

“I was just thankful that we were able to get out, go to the emergency room and survive it, because a lot of people don’t,” she said.

The colorless and odorless gas at high concentrations can knock a person out, almost instantaneously and stop their breathing within a minute.

“It would be completely preventable if we would like put detection in indoor spaces, but we don’t,” said Wilson.

There’s no national requirement for carbon monoxide alarms anywhere. Except now, for the first time in federally funded public housing.

“We’re just doing some fine-tuning as it relates to the new HUD requirement,” said Leonard Langston, Deputy Chief of Property and Asset Management with the Chicago Housing Authority.

It took multiple poisoning deaths for the Department of Housing and Urban Development to make the change.

Three million of its units nationwide now have to have carbon monoxide detectors by the end of this year.