How to Read a Material Safety Data Sheet (MSDS)

The purpose of these documents is to provide persons working with chemicals the most reliable information on the health and safety risks of the chemical or formulation as well as instructions for first aid, disposal, spill mitigation, etc. If you worked for a responsible company that uses chemicals you would be encouraged/required to review these documents for the chemicals you regularly handled. This practice is to be encouraged in the home shop as well. A MSDS for any chemical can be readily found by searching the Internet, "MSDS XXXX". Labels often have links to MSDS sheets.

While these documents are designed for ordinary mortals they are filled with incomprehensible jargon which I will attempt to decipher. We will use acetone as an example...

Acetone Material Safety Data Sheet (MSDS)

Section 1

This section provides common names for the chemical. In this case acetone is always simply called "acetone". If the material is a mixture the chemical components will be listed here, which is very handy for figuring out what is in something, for example the solvents in a formulated finish, or components in a dye stain mixture. Additionally, if the MSDS is for a product that offers no clue as to what is in it this section will divulge at least the approximate composition, for example cleaners and strippers.

Section 2

Let the jargon begin. "CAS No." is the Chemical Abstracts number assigned to this chemical. 67-64.1 is the CAS number name for "acetone". "ACGIH TLV" is the concentration ("threshold limit value (TLV))" of a chemical that a worker can be exposed to day after day for a working lifetime without adverse health effects. TLV is a reserved term of the American Conference of Governmental Industrial Hygienists (ACGIH). These data seem to be falling out of favor to be replaced by the data tabulated below it. "STEL" is the short term exposure limit, or the maximum concentration of the chemical that a worker should not be exposed to for a short time.(translation: workplace concentration must never go above this amount) It is logically higher than the "TWA" Total Workplace Average". According to OSHA, the Occupational Safety and Health Administration, "TWA is the employee's average airborne exposure in any 8-hour work shift of a 40-hour work week which shall not be exceeded."

Concentration for vapors is usually expressed as "ppm" which stands for parts per million which translates to the more comprehensible equivalent of milligrams of the chemical per cubic meter of air. To summarize these data: OSHA has determined that a worker can safely work an 8 hour shift in a concentration of 1 gram of acetone (approximately teaspoons) per cubic meter of air.

How did OSHA come up with these values? For something like acetone that is widely used in industry there is a vast body of experience with worker exposure from which to set these values. Responsible companies, for example Kodak, DuPont, etc. have measured workplace concentrations of chemicals for decades and monitored worker health to develop internal limits for exposure. These and other data were used by OSHA. For a new chemical the levels would be conservatively set from animal studies.

There follows the "Emergency Overview" which gives worst case hazards for this chemical. For acetone we are informed that it is highly flammable and if you drink or breathe enough of it it will cause the problems listed. It is well to note that this section provides worst case information, what can happen if you spill, drink or breathe enough of it. Salt can show up with alarming properties in this section. This section usually illustrates that enough of anything can be a hazard. More relevant information for typical encounters with the chemical is to be found elsewhere in the MSDS.

I regard the "Hazard Ratings" part of Section 2 as the most useful part of the MSDS for quick reference. Here we have a quick summary of the relative hazard of the chemical. We see that the health hazard for acetone exposure is slight and it is highly flammable. I interpret this summary to mean that the main concern in using acetone is high fire hazard. The other two entries are of most relevance to chemists and hard to summarize so I will skip them.

Section 3: Physical Properties

Much of these data are of little interest to the woodworker. However, "Odor Threshold" combined with the TWA given in Section 2 is relevant. In the case of acetone you can smell it at 62 ppm and 1000 ppm is safe to work in. Be especially wary of chemicals where the TWA is less than the odor threshold for in this case your nose will not provide a warning of an unsafe concentration. In contrast for acetone, and most solvents, being able to detect it by smell doesn't necessarily mean the concentration is a fire or health hazard. Odor is a poor measure of risk.

Section 4: Fire Data

Flash Point Definition. Flash point is a temperature (probably in degrees F but we don't know for sure in this case because it doesn't say). If the liquid is above this temperature a spark will ignite the vapor. We see that a spark or flame will ignite acetone at any temperature that we might be using it. There are lots of ignition sources in a shop- switches and motors- hence, the fire rating of 3 in Section 2. The "Flammable Limits" entry tells us what concentration of acetone in a vapor will ignite/explode. Almost any solvent we use in woodworking will have a similar value. Note the confusing change in concentration units. It takes some chemical gymnastics to convert from ppm to volume percent.

Section 5

This section is mostly of use to people doing chemistry with the chemical and I will skip it.

Section 6

From the Occupational Safety and Health Administration manual:

"LD stands for "Lethal Dose". LD50 is the amount of a material, given all at once, which causes the death of 50% (one half) of a group of test animals. The LD50is one way to measure the short-term poisoning potential (acute toxicity) of a material. LC stands for "Lethal Concentration". LC values usually refer to the concentration of a chemical in air but in environmental studies it can also mean the concentration of a chemical in water. For inhalation experiments, the concentration of the chemical in air that kills 50% of the test animals in a given time (usually four hours) is the LC50 value."

In the case of acetone we see that it takes a lot of it to kill rats and rabbits which is why it has a health rating of 1 in Section 2. If the chemical has any special toxicological properties, for example carcinogen, we would find that information here.

The rest of the document is self explanatory.