WASHINGTON — The Department of Housing and Urban Development will begin requiring federal inspectors to check public housing apartments for carbon monoxide detectors — but the agency won't penalize landlords for broken or missing detectors.

The move — part of HUD's effort to improve public housing conditions for millions of low-income families — quickly faced criticism from affordable housing advocates for not doing enough to protect residents from the threat of carbon monoxide poisoning.

The new requirement to check for detectors is the first concrete step that HUD has taken to address carbon monoxide hazards since an NBC News investigation revealed the lack of protections for residents of public housing. At least 13 public housing residents have died from the CO poisoning since 2003, according to NBC News' latest count, including four this year.

Under the new requirements, which take effect April 1, federal inspectors must check for the detectors in any public housing units that contain fuel-fired appliances or have an attached garage, and determine if they are working, according to a HUD notice issued Monday and obtained by NBC News.

The inspectors' findings will be “for data collection purposes only,” according to the notice: “The collection of this data does not affect a property’s inspection score.” All HUD properties are subject to routine mandatory health and safety inspections. Landlords are required to fix life-threatening hazards immediately, and properties with failing inspection scores are subject to enforcement action by HUD.

But there will be no immediate consequence for properties without working carbon monoxide detectors, because the detectors are not required by HUD.

“CO poisoning remains a serious issue in housing across the nation,” HUD said in its notice to inspectors, noting that more than 400 people die and 50,000 are sent to the emergency room every year due to carbon monoxide poisoning. “Because CO is undetectable through sight, smell, sound, or touch, a device is required to determine the presence of high and dangerous concentrations of CO gas in a residence.”

The new requirements will apply to properties owned and operated by public housing authorities, as well as privately owned developments under contract with HUD, according to HUD spokesman Brian Sullivan. But they will not apply to privately owned apartments rented to people with Section 8 housing choice vouchers.

When asked what HUD's next steps on carbon monoxide protections would be, the agency declined to say. "We’re going to speak more about CO very shortly," Sullivan said.

Members of Congress recently introduced a bill to require carbon monoxide detectors, which can cost as little as $20, in all public housing. They were heartened by HUD's action this week but said that it did not come close to fixing the deadly threat posed by the gas.

Rep. Jesus "Chuy" Garcia, D-Ill., said that HUD's new notice was "encouraging but not nearly enough."

"For HUD to allow this to fester for so many years, it’s just unconscionable," said Garcia, who co-sponsored the House version of the bill. "I can’t think of a more straightforward solution."

“I hope this is just the first step by HUD towards fixing this urgent health crisis,” said Rep. Joe Cunningham, D-S.C., who is co-sponsoring the House bill. “Determining how widespread the problem is will not save lives on its own, and I will not rest until every public housing unit has a working carbon monoxide detector.”

Sen. Kamala Harris, D-Calif., who co-sponsored the bill in the Senate, urged lawmakers to take the next step by passing the legislation: "Congress must do more now to protect families living in public housing and assure people that they and their children will be safe."

Sens. Tim Scott, R-S.C., and Bob Menendez, D-N.J., sent a letter Thursday to HUD Secretary Ben Carson asking the agency to give a timeline for its next steps.

Housing advocates also criticized HUD for not acting more quickly to protect public housing residents from carbon monoxide poisoning. HUD’s response “is overwhelmingly inadequate and does nothing to protect human lives,” said Emily Benfer, visiting associate clinical professor of law at Columbia University and a public health expert. “Where swift action is necessary, HUD proposes to wait and see or study well-settled issues.”

Deborah Thrope, supervising attorney at the National Housing Law Project, an advocacy group, agreed that the guidance “falls short of ensuring that federally assisted housing is safe for families.”

HUD has been aware of the threat posed by carbon monoxide for years.

In January 2017, an internal team dedicated to fixing troubled HUD properties presented a host of recommendations to the agency. These included examining “carbon monoxide detectors in the inspection process,” according to a report released last week by the Government Accountability Office, Congress’ watchdog agency.

But more than two years later, HUD has yet to implement any of the recommendations to overhaul the inspection process for multifamily properties, the GAO report said. HUD officials told the watchdog that the delay was due to staff vacancies in its multifamily office — a problem that’s been endemic across the Trump administration.

"There is no reason that the federal government cannot protect the most vulnerable among us when the solution is so affordable," Rep. Garcia said.