Why does UNIX 6100 mask have three sizes, while UNIX 5000/5100 has one universal size?
The answer is simple: their main difference is face seal. UNIX 6100 is available with a single face seal, because this is a lightweight model with a more compact and comfortable design. Its weight does not exceed 480 g.
UNIX 5000/5100 has a double face seal. A person with any head parameters can wear this mask: two face seal lines create a tight connection between the mask and human skin. Therefore, this product does not require several sizes.