Molten polypropylene is spun into filaments that fall straight onto a moving belt as a loose web. Heated, patterned rollers then bond the web into spunbond fabric. The outward layer is colored and made water-repellent, and the inner layer stays white and absorbent.

Melted polypropylene is forced through a die with up to 40 holes per inch while jets of hot air blast the streams. The air stretches them into fibers far finer than spunbond, which tangle on a belt into a soft, weak sheet that traps tiny particles. The fibers are often given a lasting static charge, which helps catch particles that would otherwise slip past.

Rolls of outer spunbond, melt-blown filter and inner spunbond unwind together into one three-ply web. The colored side faces the world and the white side faces the wearer.

Guides fold the web into pleats running across the mask. Unfolded, they let the mask stretch from the bridge of the nose to below the chin.

A horn vibrating at ultrasonic frequency presses on the edges and melts the plastic layers together without thread or glue. It adds no contamination, which matters for medical goods.

A small bendable metal strip is sealed along the top edge so the mask can be pinched to the nose. Elastic ear loops or tie straps are welded to the four corners.

Sample masks face a spray of particles carrying bacteria, about 3 micrometers across. Under the European Type II standard the material must cut the bacteria that get through by at least 98%.