The usual way of measuring the safety of solvents is to specify how many parts per million of the vapor in the air a person can take without ill effects. This assumes that the most frequent kind of exposure is through the lungs, though it is also true that solvents can be absorbed through the skin and mouth. The Time Weighted Average (TWA), in parts per million (ppm), is the figure to use if you are going to be working with a solvent all day. The Short Term Exposure Limit (STEL), also in ppm, is just what it sounds like.
Eleven of the most frequently used solvents for book and paper conservation are listed below. Although they were also selected for safety, this does not mean they can all be used without a fume hood or other form of protection. Some of then (e.g. ethyl acetate and acetone) are highly flammable. Anyone who uses these or other chemicals should have and use reference books or data sheets that give all relevant information about each chemical.
Acetone - STEL was recently lowered from 1,250 to 250 ppm.
Cyclohexane - STEL 375
Ethanol (Ethyl alcohol) - TWA 1000; no STEL set
Ethyl acetate - TWA 400. Flash point under 80°F.
Heptane - STEL 500
Isopropanol (Isopropyl alcohol) - STEL 500
Methanol - STEL 250
Methyl ethyl ketone (MEK) - STEL 300
Methylene chloride - STEL 500
Propanol (Propyl alcohol) - STEL 250
Tetrahydrofuran - STEL 250
Note that the lowest figures are for the more dangerous solvents. If no limit is set, no hazard is perceived.