Tag Archives: ventilation

Safety

Smell is Irrelevant

“This doesn’t smell bad so I don’t need a respirator.” I hear that from time to time, either from students or in online forums. Prop makers working with chemicals use their sense of smell to determine how dangerous something is. “This smells better than that, so I don’t use that anymore.” “I can’t smell a thing, so this must be safe.”

No no no. This is dangerous, and the wrong way to think about safety with chemicals.

For every chemical, OSHA sets limits as to how much you can be exposed to. They try to figure out the amount you can be exposed to while working with something your entire life, and never have adverse health effects from it. These are called Threshold Limit Values (TLV).

The first is  the time-weighted average (TWA). The TWA is meant to indicate what you are constantly exposed to at work. They measure the average amount you are exposed to over an 8-hour day and a 40 hour week. (Uh oh, we often work much more than that in theatre).

Next is the short-term exposure. or STEL. They define this as 15 minutes of exposure. And you have to have an hour break before the next exposure. And you can only have four exposures per day.

Finally there is the ceiling value. You should never reach this level of exposure, even for an instant. They also have IDLH, which is “immediately dangerous to life and health”. Instant exposure at this amount will kill or irreversibly affect your health.

So let’s look at the following chart, which has the TLVs for some common chemicals found in the props shop. You probably recognize some of these as ingredients in paints and coatings. Amines are found in many epoxies. Methyl ethyl ketone is used in polyester resin. The diisocyanates (TDI and MDI) are two of the more common curing agents used in two-part polyurethanes.

All the values are measured in parts per million (ppm), which means out of a million pieces of air, that is how many pieces are of the substance being measured (for comparison, room air has 209,500 ppm of oxygen).

Odor thresholds and Threshold Limit Values of certain chemicals

So where does smell come in? Well, every chemical has an “odor threshold”. This is the amount, again in ppm, of a chemical at which point you can smell it. This is much less standardized, because it can be hard to test and different people have different sensitivities to smell. The number is often given in a range. You can see in the chart that the odor thresholds are all over the place for the different chemicals.

I’ve pulled a few chemicals out and put them in a chart so you can see what’s happening a bit easier.

 

Chart for OT and TLV

Look at chlorine. The odor threshold is way below the TLV TWA. This means that even if you smell chlorine, you may not be exposed to a harmful amount. You may be able to smell chlorine all day every day and still not have harmful effects (like if you work at an indoor pool).

Now look at formaldehyde. The short-term exposure limit (the orange dot) is way down in the graph. The odor threshold is way at the top. That means you can be exposed to a harmful amount before smelling it. In fact, you will have to be exposed to three times the threshold limit before you can smell it. So if you are working with something that off-gasses formaldehyde (including many plywoods and engineered lumber, VOC-containing paints, and even some fabrics), you cannot assume you are safe because you do not smell anything.

Look at the two diisocyanates (MDI and TDI). Both of them also have an odor threshold above their short-term exposure limit. If you look back at the chart, you will see that the STEL for MDI is also its ceiling value, which is the amount you should not exceed even for an instant. And its odor threshold is twenty times higher than that. You can be breathing dangerous and even deadly amounts of MDI before you even get close to smelling it.

This is why many people suggest casting urethane parts inside a fume hood or a spray booth; even a respirator is not a reliable protector. One way to tell if your respirator has stopped working is if you can smell the outside air. But with these chemicals, you cannot smell them even when they are present in dangerous amounts. So you have no indication of whether your respirator is working or not.

Your nose is a great sensor for many chemicals, but you should never rely solely on it for your safety. You need to know about the specific chemicals you are working with and how their odor threshold relates to their threshold limit values. No more, “this doesn’t smell bad so I don’t need a respirator.”

Smell you later.

Safety

The Nose Knows Not

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I often see a lot of products advertise themselves as “low odor”. I also hear the occasional prop maker mention that one product is safer because it “smells better” than an alternative.

What is smell? Smell means you are detecting airborne particles, fumes, gases, vapors, dusts and mists. And if these tiny airborne things are reaching your nose, than you can be sure some of them are entering your lungs, and from there, your bloodstream. So smelling something is a warning that you may be breathing hazardous substances.

But the smell is not related to the toxicity of that substance. In fact, particularly odorous substances can, in some situations, be safer than their low-odor counterparts. Relying on your sense of smell is a poor method of determining the quality of the air you are breathing and whether you should be wearing a respirator or working in a spray booth. Let’s see why.

First, a brief foray into the world of measuring toxic exposure amounts, as well as how we measure “smell”. You need the MSDS to know what chemicals are in the products you are using and in what quantities.

OSHA measures the amount of a substance in the air using “parts per million”, or PPM. For example, if Chemical X is recorded at 1000 PPM, than for every million atoms of air in a room, one thousand of those are Chemical X. The other 999,000 are probably atoms of oxygen, nitrogen, carbon dioxide, water vapor and so forth.

To determine the safe level that certain chemicals can be worked at without causing harm, OSHA has a number of measurements related to the threshold limit value (TLV). The TLV gives a number in PPM; above that number is harmful, below is not. The TLV is indicated in a number of ways. There is the “ceiling value”, or TLV-C, which is the amount that should never be exceeded. The TLV-C is usually pretty high, because it takes a lot of any single chemical to harm you in one breath. More common is the time-weighted average (TLV-TWA). This gives you the average level of a chemical exposure over a period of time (usually eight hours unless otherwise indicated).[ref]You will also run across the PEL (Permissible Exposure Limit) of a chemical. This is the actual legal limit established by OSHA, above which an employer cannot expose its workers to. You have to check what the PEL is measure in; a TLV-TWA for eight hours is often used, but it may also a shorter exposure time or even a ceiling limit.[/ref] This number is far lower than the TLV-C, because you are being continuously exposed to a certain level over an extended time.[ref]Many other organizations have their own standards and measurements, and not every chemical has been measured in every way. So acetone has a TLV-TWA of 500 PPM, but the TLV-C has not been established by OSHA. It does, however have an IDLH (Immediately Dangerous to Life or Health) of 2500 PPM; this is typically a bit stronger than TLV-C, indicating you can probably die with a short exposure (under 30 minutes) at this level.[/ref]

Let’s look at acetone. Acetone has a TLV-TWA of 500 ppm. That means that over an eight hour day, your body has been harmed in some way if you have breathed, on average, 500 molecules of acetone with every million pieces of air. It may be higher at times—such as when you open a can of acetone—and lower at other times, such as when it has all evaporated and you are working on something else.

Getting back to smell, the other important measurement is the Odor Threshold (OT). This measurement, also in PPM, indicates at what concentration you can smell that particular chemical. Acetone has an OT of 62 PPM.

Let’s see what happens. You are in your shop working with acetone. It fills the air at 30 PPM. You keep working with it. It is now 62 PPM; you start to get a whiff of that distinctive acetone smell. “Uh oh.” you think. “Better open a window and set some fans up.” The increased ventilation brings the concentration of acetone back down to 50 PPM. You no longer smell it. During this whole time, your exposure to acetone never even gets close to 500 PPM because the smell alerts you to the fact that you are being exposed; you smell it in a concentration far below what is dangerous to breath.

Now let us look at another chemical common in the props shop. Hexane (or n-hexane) is used as a solvent, and is found in some cleaners and degreasers, as well as in adhesives, particularly fast-drying glues or cements intended for leather. Hexane has  a TLV-TWA of 50 PPM and an OT of 130 PPM.

Let’s step through another typical day. It’s the morning and you are gluing some leather together. Your exposure to hexane creeps up to 80 PPM for a few minutes. You clean something off with a hexane-containing cleaner and the concentration of hexane goes up to 100 PPM. You work on something else for a few hours and the level of hexane drops to 10 PPM as it evaporates. In the afternoon, you are using some rubber cement and white-out (both of which typically contain hexane) and your exposure goes back up to 60 PPM. In fact, by the end of the day, your average exposure (your TLV-TWA) has been around 55 PPM—above the limit of 50 PPM, meaning you inhaled a harmful amount. However, the level never even approached the OT of 130 PPM, so you never smelled it.

In other words, if you relied on your sense of smell to warn you of dangerous chemical exposure, it would have failed you in this case.

Any chemical with an OT above its TLV-TWA will not warn you with its scent before you are exposed to dangerous levels. Some chemicals lack any adequate warning signs for overexposure. The cyanates used in polyurethanes popularly used in molding and casting are particularly egregious. For instance, Methylene diphenyl diisocyanate (MDI) is commonly found in two-part rigid polyurethanes and polyurethane foam. Though one of the least toxic of the isocyanates, it still causes harm at low levels[ref]See this compilation of health hazards of MDI.[/ref]. Exposure can also create sensitization or allergies, which leads to violent or even fatal reactions in workers exposed to even a small amount. The TWA is only 0.005 PPM (the PEL is actually 0.02 PPM, but that is a ceiling limit). The OT has not even been established, but you can be expected to have some warning in the form of eye and nose irritation around 0.05 to 0.1 PPM. [ref]Occupational Health Guideline for Methylene Bisphenyl Isocyanate (MDI), US Dept of Health and Human Services, 1978.[/ref] In other words, you may not have any warning until you have been exposed to at least ten times over the amount that is safe to breathe over eight hours, or even five times the maximum amount you should breathe at any one time. Even then, you may not correlate your runny nose or watery eyes to the polyurethane; it has no distinctive smell, so you may just continue on, thinking “Hey, this is great. It doesn’t smell bad, so it must be safe to breathe.”

That’s wrong. Dead wrong.

Safety

The Well Ventilated Shop

I am back in New York City for a few weeks; between packing and traveling, I have not had much time to write. I did want to share the following quote, however. It comes from an article titled “The Construction of Theatres”, and it is written by Warrington Taylor.

Then the property and armoury rooms must be near the stage, and a very well ventilated property shop, for the making of green leaves and other necessaries is a most unhealthy employment.

The article was written in 1865. Most of our prop shops have been built well after that, yet so many still suffer from bad or no ventilation. How can there be any excuse for this? No one should be able to run a shop or a theatre without being aware of this simple fact. We like to imagine that we know much more about safety and chemicals these days; while it is true that you can find old timers who used Celastic without gloves and worked with asbestos without a mask, the simple fact is that people were well aware of the fact that prop shops require good ventilation almost 150 years ago.