The state’s cache of 21 million face masks is so old that it can’t be relied on to protect health care providers scrambling to deal with the coronavirus, NBC Bay Area’s Investigative Unit has learned.

The virus has triggered national shortages of N95 respirator masks – so named because they filter out 95% of particles.

Last week, state officials said they had gotten approval from the Centers for Disease Control and National Institute of Occupational Safety and Health to tap the 21 million “emergency planning reserves’’ masks in “certain situations.”

That approval, the state Department of Public Health acknowledged, was sought because the state’s stockpile of masks is so old they can be prone to failure. The masks themselves don’t break down, but the elastic straps that secure them to the face can become brittle after the expiration date and snap during use, potentially exposing a medical professional to disease particles.

While state officials would not say just how much of the cache is expired, sources tell NBC Bay Area that the number of unexpired masks amounts to just a few hundred out of the 21 million kept across the state.

While state health officials touted their efforts to store masks in such a way to preserve the elastic from deteriorating, the state acknowledged the Centers for Disease Control and national occupational health officials granted their use only in “limited, low-risk circumstances, thus relieving pressure on the supply chain of unexpired masks” for use in medical treatment.

Public Health Department Director Dr. Sonia Angell said in a statement that “releasing this supply of masks will help keep our health care professionals safe on the job.”

State officials did not answer questions about how they have been managing their stockpile and relying on expired masks in lieu of routinely refreshing their supply.