Four numbers
On this page11
The permissible exposure limit
No employee exposed above "0.75 parts formaldehyde per million parts of air (0.75 ppm) as an 8-hour TWA". That is the number the whole standard is built around.
The short term limit
No employee exposed above "two parts formaldehyde per million parts of air (2 ppm) as a 15-minute STEL". A short, sharp exposure has its own ceiling.
Which is the one a classroom hits
Opening a specimen jar is a fifteen minute event, not an eight hour one. The STEL is the limit a dissection lesson is most likely to approach.
The action level
"A concentration of 0.5 part formaldehyde per million parts of air (0.5 ppm)" as an eight-hour TWA. Below the PEL, and it is the level at which obligations begin rather than the level at which they are breached.
So there are three airborne numbers, not one
An operation can be under the PEL and over the action level, which is a different set of duties. Reading only the 0.75 figure misses most of the standard.
And one composition number
The hazard communication requirements reach "all mixtures or solutions composed of greater than 0.1 percent formaldehyde, and materials capable of releasing formaldehyde" at 0.1 ppm or above.
Capable of releasing is the phrase that matters
A preserved specimen is not a solution of formaldehyde; it is a material capable of releasing it. The standard reaches it through that clause rather than through its composition.
Which is why the container is the control
Every one of the three airborne numbers is a measurement of air. What keeps the air clean is a vessel that seals, opened deliberately, in a room with ventilation.
A loose lid is an exposure
Not a mess. A specimen jar that weeps vapor into a cupboard raises the eight hour average for whoever works in that room, which is the teacher rather than the class.
And the cupboard is part of the system
Thirty jars in an unventilated store is a different exposure from thirty jars in a fume cupboard. The container choice and the storage choice are one decision.