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Written by Robert E. Sheriff, MS, CIH, CSP, President
October 24, 2019
Local Exhaust Ventilation to Control Air Contaminants
Local Exhaust Ventilation for reducing worker exposure is the best description of ventilation for the purpose of controlling air contaminants that may be released inside the building. The objective is to capture the containment before it reaches the operation and gets into the general air stream. As of 2017, Industrial Production has increased but corresponding efforts to control worker exposure from the emissions from that production have not kept pace. Thus, workers are being exposed to contaminants in their work environment as the demand for American made products increases.
There is a hierarchy of controls to minimize the potential for worker exposure to workplace health hazards where the exposure is through inhalation. This applies to both solids (lead, dust, etc.) and chemicals (vapors, all gases, solvents, acids, pesticides, etc.).
The hierarchy of controls includes:
- Eliminates the Hazard – find a substitute that is less toxic or non-toxic.
- Separate the worker from the contaminant. Isolate the worker from the contaminant or enclose the activity to eliminate exposure.
- Local Exhaust Ventilation – Capture the contaminant before it gets into the worker’s breathing zone.
- General Ventilation – Move the contaminant elsewhere or dilute it to reduce or eliminate worker exposure. Be careful not to just send the contaminant into other workers’ breathing zones.
- Administrative Controls – Minimize the worker’s exposure through a reduction in exposure time. Possibly split work activity with other workers to reduce the time one (1) worker is exposed.
- Respiratory Protection – The most difficult—and generally least effective worker exposure control is respiratory protection—for many reasons:
- The correct respirator must be selected that is specific for the specific contaminant and the concentration of the contaminant.
- Each person’s face is different and the effectiveness of the respirator’s fit can vary considerably.
- Persons must be properly trained in how to wear the respirator.
- The respirator must be inspected, cleaned, properly stored and filter change at proper intervals.
- The worker’s environment may make it difficult to wear. Too Hot, Too Cold, Too Humid, Too Much Motion. The need to communicate with others.
- The wearer must have a well-functioning respiratory system that allows for wearing a respirator for part—or all—of a work shift.
Local Exhaust Ventilation
From an engineering standpoint, local exhaust ventilation is the best approach. Local exhaust is intended to locate the pick-up point of the exhaust close enough to the work activity to draw the contaminant away from the worker’s breathing zone and into a system that can capture and filter the material.
This can apply to such diverse activities as:
- Laboratory hoods
There are specific design parameters to consider: 1) the closeness of the exhaust point, 2) the weight of a particulate, 3) the vapor pressure-(evaporation rate) of the vapor, 4) the temperature of the material (heat rises) 5) the transport velocity—keep it suspended in the ductwork, 6) the filtration system (filters or chemical absorbers), 7) the size and type of fan system, 8) the location of exhaust to the atmosphere (not near air intake), 9) will an air permit be required?
The design must also consider make-up air which often must be heated or air-conditioned to maintain a comfortable—or at least tolerable—temperature. You can’t just add exhaust ventilation without considering the need for wake-up air. If not, the exhaust system will not be able to capture the needed amount of air or the negative pressure will get so great, doors can’t be opened and there are temperature gradient problems throughout the building.
General Ventilation must also be considered since make-up air and exhaust (whether local or general) need to be balanced. General Ventilation is usually described in terms of AIR CHANGES PER HOUR. That is, how often is the total amount of air supplied/exhausted compared to the volume of the building’s space. The following examples of some recommended Air Exchange Rates in Air Changes Per Hour:
|All Spaces in General||4 air changes per hour (ACR)|
|Churches||8 – 15|
|Factory Buildings – Ordinary||2 – 4|
|Factory Buildings – Fumes and Moisture||10 – 15|
NOTE: These air changes can include the local exhaust rates for specific operations.
There are a wide variety of approaches to Air Contaminant Control for Worker Protection. The following is a general discussion of some of the more prevalent situations where local exhaust ventilation is the preferred method of Contaminant Control:
For stationary welding on a bench or even assembly line, a backdraft hood is generally the best approach. Where possible, side baffles are ideal to control airflow from behind the worker into the backdraft exhaust system.
For mobile welding points, an “Elephant Trunk” type exhaust can be moved near the weld to capture the fumes before it reaches the welder’s breathing zone. Special care must be taken on this type of exhaust not to place the exhaust’s opening so close to the weld point that it interferes with the inert gas shield around the weld itself. The inert gas must be maintained so that the weld is not oxidized resulting in defective welds.
Whether the paint booth is a walk-in type of benchtop, a backdraft system is the appropriate means to draw the particulates and solvents away from the worker’s breathing zone and into the ventilation system. It is important to maintain a minimum airflow at the worker’s position—called the “capture velocity.” This rate of airflow for general purposes should be at least 75 to 100 feet per minute. It is also critical that the booth operator not stand in the airflow’s stream between the spray gun and the exhaust. This will result in drawing the paint emissions into his breathing zone and actually increasing that person’s exposure.
Saws and Grinders
For portable saws and grinders, some equipment is actually designed with an attached local exhaust. This is very effective when the exhaust is located in the air stream of the particulates. (You’re using the particles’ momentum to aid in its capture). It is also beneficial because it uses a small volume of air—saving heat, air conditioning, and electricity.
For stationary saws and grinders, either a backdraft or downdraft hood will capture the particulates before they reach the operator’s breathing zone. Exhaust attached directly to the equipment—as with portable units—can also be very effective. Control of the direction of airflow can be enhanced by installing side panels and top panels to the table or bench. When only particulates are involved, the exhaust can be placed very close to the generation point. The closer the exhaust the less air volume that is needed for effective capture.
Dip Tanks and Plating Tanks
Proper contaminant control can require more sophisticated containment systems. Material movement and rising fumes from heated liquids can cause special problems. One solution is a push-pull system where a slotted backdraft hood is behind the tank and slotted air supply is in the front of the tank “pushing” the air over the tank’s surface toward the exhaust, a very delicate system but very effective.
These are just a few of the types of Local Exhaust/Containment Control Systems effective in controlling worker exposure to workplace hazards.
SPECIAL NOTE ON CANOPY HOODS
Although it may appear logical to install a hood directly over a work activity to capture the heated or evaporating materials, they are not generally recommended. Primarily due to the worker having to stand under the hood, thus, drawing the material into his breathing zone rather than away from it. Also, canopy hoods when set close to the emissions and the worker, they cannot safely perform the required tasks. Canopy hoods are only recommended where a worker does not have to work over the emissions source; example, a robotic spray operator or a mechanical lid/cap applicator.
Air Contaminant Control Ventilation is a complex set of factors, formulas, calculations, and designs that require input from a variety of professionals: Engineers, Industrial Hygienists, Safety Professionals, Ergonomists and production personnel.
One of the best available references is the text: Industrial Ventilation: A Manual of Recommended Practice for Design. Now the 29th edition published by the American Conference of Governmental Industrial Hygienists (ACGIH) in 2016.
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