They lie carelessly on streets, hang in hedges or swim in rivers.
The disposable medical mask - a key element in the fight against the corona pandemic - has become a serious environmental problem.
Danger looms from the rubber bands, in which animals can easily get tangled.
Because the masks are made of the persistent plastics polypropylene and polyester, it takes decades for them to decompose.
Once in the sea, they multiply the plastic waste floating around.
In the end, they may end up as microplastics in the food chain of many sea creatures, so the fear.
Throw it into the fire is therefore good advice.
According to the Federal Ministry for the Environment, the face masks belong in the residual waste after use anyway.
Chemists from Washington State University in Pullman are now showing that there is another way.
Xianming Shi and his colleagues suggest adding the components of the face masks to cement to make concrete more stable.
The researchers reveal their recipe in the "Materials Letters": First disinfect masks and remove cotton loops and metal parts, then shred the mask fabric and treat it chemically until it breaks down into small polypropylene and polyester fibers;
The fibers are then coated with graphene oxide, mixed with Portland cement and finally processed into concrete.
According to Shi and his colleagues, the result was convincing: after hardening, the concrete became significantly stronger than without the addition of fibers.
The fibers would dissipate the fracture energy and thus prevent cracks in the material.
For this reason, steel, plastic or glass fibers are already added to some types of concrete.
Microfibers from masks could open up a cheap alternative.
It remains to be seen whether the method developed by Shi and his colleagues is actually suitable for practical use.
In any case, the work of the American recycling specialists gives hope that the corona waste can somehow be brought under control.
In any case, the replenishment of masks should not break off so quickly, despite the first easing.