At the start of the crisis, an HHS spokeswoman said the Strategic National Stockpile had about 13 million N95 respirator masks, which filter out about 95% of all liquid or airborne particles and are critical to prevent health care workers from becoming infected. That's just a small fraction of what hospitals need to protect their workers, who normally would wear a new mask for each patient, but who now are often issued only one to last for days.