It is hard to conceive of the size of a fiber optic connector core. Single mode fibers have cores that are only 8~9µm in diameter. As a point of reference, a typical human air is 50~75µm in diameter, approximately 6-9 times larger.
Dust particles can be 20µm or larger in diameter. Dust particles smaller than 1µm can be suspended almost indefinitely in the air. A 1µm dust particle landing on the core of single mode fiber can cause up to 1dB of loss. Larger dust particles, 9µm or larger, can completely block the core of a single mode fiber.
Such that fiber optic connectors have to be cleaned each time they are mated. It is essential that fiber optics users develop the necessary discipline to always clean the connectors before they are mated.
The other important thing to keep in mind is that you need to cover a fiber optic connector when it is not in use. Unprotected connector ends are most often damaged by impact, such as hitting the floor. Most connector manufacturers provide some sort of protection boot. The best protectors cover the entire connector end, but they are generally simple closed-end plastic tubes that fit snugly over the ferrule only.
These boot will protect the connector’s polished ferrule end from impact damage that might crack or chip the polished surface. Many of the tight fitting plastic tubes contain jelly-like contamination (most likely mold release) that adheres to the sides of the ferrule. A blast of cleaning air or a quick dunk in a alcohol will not remove this residue.
This jelly-like residue can combine with common dirt to form a sticky mess that causes the connector ferrule to stick in the mating adapter. Often, the stuck ferrule will break off as one attempts to remove it, so always thoroughly clean the connector before mating, even if it was cleaned previously before the protection boot was installed.