By now, you probably know more about N95 respirators than you thought possible. In the midst of the coronavirus pandemic, these masks have become lifesaving tools to medical personnel and other frontline workers in the fight against COVID-19. But lately, you may have spotted ads for N99 or R95 masks.

No, that’s not a typo. Here’s a short guide to what they are.

What is a respirator?

Let’s start with the basics. All of these masks are respirators, rather than surgical masks. While surgical masks fit loosely on the face, allowing particles to enter through the sides, respirators are designed to create a seal around the face so they filter out most particles. Respirators provide the best possible protection in the midst of the current crisis, which is why governments and hospitals have been racing to buy them for frontline workers. This has led to a global shortage of N95 masks.

What do the numbers and letters mean?

Respirator names consist of a letter and a number. The letter—N, R, or P—refers to whether the mask is resistant to oil-based particles such as solvents and pesticides. (N is non-resistant; R is oil-resistant; P is oil-proof, so totally resistant to oil.) The number refers to what percentage of airborne particles the mask filters out. The R and P masks are largely geared toward industrial workers; oil resistance isn’t particularly relevant right now since coronavirus particles aren’t oil-based.

Generally, the more particles the respirator filters out, the harder it is to breathe when wearing the masks because they let in less airflow. Some respirator masks are equipped with a one-way valve that allows the wearer to breathe out, but filters out particles that are in the air breathed in. This isn’t a great option in the current crisis because if you happen to be an asymptomatic carrier of the virus, you would be breathing out virus-laden particles. Without this valve, many people report that it’s hard to wear higher filtration masks for a long time without feeling suffocated. Even N95 masks are meant to be worn for a limited period of time: After a while, particles get stuck inside the filter, making it harder for the wearer to breathe.