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Some Common
Ventilation Foul-Ups

UE News, April 1999

Ventilation foul-ups ...

In most industrial plants, there are really two different ventilation systems: the general room ventilation system and a local exhaust ventilation system. If not properly balanced and maintained, these two systems can interfere with each other, causing workers to breathe in heavy doses of toxic dusts, vapors and fumes.

The purpose of general room ventilation in a plant is primarily to provide a comfortable work atmosphere, especially to keep the plant cool in the summer, and secondarily to slowly remove small amounts of toxic materials from the workroom air. This system is typically controlled by exhaust fans on walls and ceilings, and air circulation fans on the shop floor.

The local exhaust system, on the other hand, is designed to capture airborne dusts, fumes and vapors coming from a particular "hot" source or process, by putting a special ventilation hood close to the source and drawing up the air and toxics right out of the plant.

These two systems are supposed to operate more or less separately, and often at first they do. But as industrial operations change over time, or as changes are made to the building (such as new walls, new workrooms, new offices) one ventilation system can interfere with the other. The result can be more worker exposure to toxic substances rather than less. Let’s look at some of the common examples of interference between these two ventilation systems and how they can be prevented.

  • Suppose you are welding or mixing chemicals on a table, and there is a ventilation hood overhead which is designed to draw up and catch the fumes and vapors. Now suppose one of the fans from the room ventilation system (or just an open window or door) creates a cross-draft that blows the fumes away from the hood and toward your face. This can result in a dangerous toxic exposure, especially if the fumes or vapors don’t have a noticeable smell. This can also happen when the overhead ventilation hood gets clogged with dust and can’t draw the fumes up efficiently.

What to do? First, check for cross-drafts before you start the job. One simple way to do this is to turn on the local ventilation system, put a lighted cigarette on the work table and see if its smoke is drawn up into the hood as it should be. If some or all of the smoke misses the hood, look around for fans or open doors or windows which might be causing the cross-drafts. If you make the necessary changes and the problem persists, check whether the air filter in the ventilation pipe connected to the hood is clogged with dust, or whether the ventilation pipe itself is clogged. If so, press management to clean or change the filter, and clean out the clogged pipes.

  • Another common problem occurs when the local ventilation unit is so powerful that it draws in general circulating room air. In this case, if the room air in the neighborhood of the worker has a relatively high concentration of toxic vapors (say, from painted or glued objects drying nearby), then the worker will breathe them in and be exposed to a health hazard.

What to do? When a new local ventilation unit is installed, check that it isn’t so powerful that it draws in lots of general room air. Again, you can check this with a lighted cigarette. Put this cigarette down at several places near the hood, especially at places where vapors might be generated (say, nearby where paints are drying, or solvent glues or epoxies are often used). If the cigarette smoke drifts into the hood rather than the overhead exhaust fans, then the two ventilation systems are out of balance and need to be modified. A variety of steps can be taken. The company can improve general room ventilation by putting in a more powerful overhead exhaust fan. It can decrease the draw of the local exhaust hood by adjusting or replacing its fan. Or it can put up some sort of barrier to cut down general room air flow into the exhaust hood. Once this problem is identified, the company probably will have to call in an outside consultant firm for detailed advice.

Remember, no matter how well a ventilation system was designed initially — and some were slapped together without a clear plan — plant operations change over time. If the general room ventilation system and the local exhaust system are not in proper balance, ventilation systems can make health conditions in our plant worse rather than better.

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