There are three reasons for ventilation: 1) for toxic airborne chemicals, 2) to prevent a build-up of flammable gases or vapors, and 3) for comfort of the inhabitants of the area. Since health effects of chemicals occur at air concentrations well below the lower explosive limits of solvents and gases, then if one ventilates to prevent health effects, one also prevents a buildup of vapors that could catch fire or explode.
There are two types of ventilation for toxic substances: dilution ventilation, and local exhaust ventilation. Dilution ventilation involves bringing in clean air to dilute the contaminated air, and then exhausting the diluted air to the outside via exhaust fans. An open door or window, or recirculating air-conditioning system is not adequate dilution ventilation for toxic gases and vapors. Local exhaust ventilation involves trapping airborne contaminants at their source before they contaminate the air which is breathed. Examples include spray booths, and dust-collecting hoods.
Dilution ventilation does not eliminate exposure to toxic gases and vapors. It lowers the concentration, hopefully to a safe level if there is a high enough exhaust rate. Dilution ventilation should not be used to exhaust large amounts of toxic solvent vapors, or for highly toxic solvent vapors, because of the requirement for large amounts of makeup or replacement air to replace the air being exhausted. This makeup air has to be heated or cooled to a comfortable temperature.
Dilution ventilation should also not be used for dusts or fumes because of the difficulty of calculating the amount of dilution air required. The exhausted air should be completely exhausted to the outside and not recirculated.
For solvents, the amount of dilution ventilation required can be calculated by the following formula: total amt evaporated (pints) x dilution volume/pint x K number of minutes
• The dilution volume is the number of cubic feet of exhaust air required to dilute the vapor concentration from the evaporation of one pint of a solvent to the Threshold Limit Value (the TLV is the 8-hour average concentration that is supposed to be safe). See Table 1 for dilution volumes for common solvents.
• K is a safety factor to allow for uneven mixing of air, being close to the point of solvent vapor production, solvent toxicity, and rate of evaporation. I normally recommend a safety factor of 10, especially since there is controversy about the safety of many TLVs.
• number of minutes is the time over which solvent evaporation occurs. For example, if 1 pint of mineral spirits is evaporated over a 4-hour period, then the amount of dilution ventilation required would be: = 1 pt x 35,000 cu. ft/pint x 10 / 240 minutes = 1,458 cubic feet/minute (cfm)
Local Exhaust Ventilation A local exhaust system consists of a hood to capture the contaminants, ducts to transport them to the outside, an exhaust fan to move the air, and sometimes air cleaners to remove particulates from the air. The only air cleaners I would recommend are filters in spray booths, and dust collectors for woodworking and other dust-producing machines. Charcoal filters are not recommended because of large amounts of charcoal required and the difficulty of telling when the charcoal is saturated. Particular types of hoods are used for particular operations. OSHA requires local exhaust ventilation for abrasive blasting, grinding, polishing and buffing, spray finishing, and open surface tanks (23 CFR 1910.94). Examples of typical local exhaust systems for art operations include overhead canopy hoods over electric kilns and glassblowing furnaces, slot exhaust hoods for cleaning etching plates and other localized operations, enclosed hoods for acid etching, spray booths for spraying paints or glazes, movable exhaust hoods for welding, and dust-collecting hoods for woodshops. (See Figure 3).
Often, either a slot exhaust hood or enclosed hood can provide adequate local exhaust ventilation. If practical, an enclosed hood requires a lower exhaust rate, and therefore less makeup air, and is also more effective. For example, a 3-foot slot exhaust hood would require an exhaust rate of 1050 cfm. By comparison, an enclosed hood with a 3-foot by 18-inch (1.5ft) opening would require only 360 cfm. Thus an enclosed hood, if practical for the type of work being done, can help reduce energy costs for makeup air.
Rules for Good Ventilation • Use local exhaust ventilation when possible.
• Provide adequate makeup air. Ensure that air intakes are not located near truck loading platforms, exhaust air outlets, furnace chimneys, etc. This makeup air should not enter the room close enough to the exhaust hood to create turbulence and affect the hood's capturing contaminants.
• Direct the flow of air so that clean air passes your face before becoming contaminated and being exhausted. (See Figure 1).
• Place local exhaust hoods or exhaust fans as close to the operation as possible.
• With local exhaust, enclose the process as much as possible.
• Local exhaust fans should be located outside so all ducts are under negative pressure, and to decrease noise levels.
• Do not recirculate any of the exhausted air.
• Make sure exhausted air cannot reenter the area (or other areas).
• Always test the exhaust systems when it is installed. This should include smoke tube (or even soap bubbles) observations at hood openings to ensure adequate capture of contaminants and where you work to ensure the contaminants are being pulled away from your face. The engineer should instruct individuals responsible for the system completely about operation and maintenance before signing off on the project.
• Ducting should be round not rectangular, and have as few elbows as possible to reduce friction. The angle of the bends should be gradual, not sharp. If needed, ducting should be corrosion-resistant (e.g. acids).
• Spark-proof construction of exhaust systems and placing fan motors outside the airstream is important for all local exhaust ductwork systems exhausting flammable gases and vapors. (See below for further discussion).
• Provide regular maintenance and inspection.
There are two basic types of fans: propeller fans and centrifugal fans. (See Figure 2). Propeller fans are the common fans we see every day and are often used in windows, walls and ceilings to provide dilution ventilation. If you use a propeller fan for dilution ventilation in a room, place it up high. Otherwise good mixing of air can be hampered by pieces of furniture that might block free flow of air.
Propeller fans can also be used to provide semi-local exhaust ventilation for small scale operations. If you place your work bench against a window, and put the propeller fan in the window at work level, it can provide good exhaust for many common operations involving small amounts of solvents and other toxic chemicals. Of course, make sure the contaminated air you have exhausted can't reenter through another nearby window or discharge onto an area where people are present. Propeller fans are normally not recommended for systems with ducts because they cannot exhaust air effectively when there is resistance to air movement caused by hoods and ducts. For local exhaust systems, centrifugal fans are used. The fans in window air conditioners, for example, are centrifugal fans.
Choosing the right type of fan can be very complicated and require the assistance of an engineer. In selecting an engineer to design a ventilation system for toxic substances, it is important to choose someone experienced in industrial ventilation. Most heating, ventilating and air-conditioning engineers do not have this experience.
Ducts transport contaminated air away from the work area. Ducting should be as short as possible, as increased in length means an increase in duct resistance, and may require more powerful fans or additional booster fans. In general:
• Round duct should be used rather than rectangular ones to minimize turbulence. • Duct turns and bends should always as gradual as possible. The angle of duct bends should be greater than 90ø so as to reduce air flow resistance. • Changes in duct diameter should also be gradual to reduce friction and turbulence. • Duct joints should be smooth and leak-free. • If dusts are being ventilated, the duct velocity must be adequate so that the dust doesn't settle and clog up the system. (e.g. 3500 feet per minute). Clogged ducts result in increased resistance to air flow, and can be a fire hazard if flammable or combustible materials are being exhausted. • If the fan and duct system supplies air to more than one local exhaust hood, then the airflow should be balanced through each branch. Rebalancing will be necessary if ductwork or hoods are added to the system. • Ducts should be inspected regularly for leaks, damages, and routinely maintained and cleaned.
Air Purifiers and Filters Air cleaners such as filters and scrubbers are sometimes used in conjunction with ventilation systems. Local codes may regulate air emissions and require filtration of exhaust air. Check your local Department of Environmental Protection for local regulations. Filters need to be regularly inspected and maintained.
Air purifiers such as electrostatic precipitators or ones with HEPA filters can capture small amounts of particulates (e.g. cigarette smoke, pollen, etc.), but are not effective against solvents and gases. The newer molecular absorbers and activated charcoal can remove small amounts of odors (e.g. kitchen odors), but are not effective for larger amounts. Regular inspection and maintenance of all air purifiers is necessary.
Fire and Explosion Hazards Flammable and combustible materials can ignite in the presence of an ignition source such as a spark. Flammable solvents, combustible organic dusts, metal dusts, or ignitable fibers are examples of such materials. Flammable solvents are a fire or explosion hazard if the solvent vapor concentration in air is greater than the lower explosive limit (LEL) or less than the upper explosive limit (UEL). Below the LEL, the mixture is too lean to burn. For example, the LEL of ethyl alcohol is 3.3% (33,000 parts per million of air), while the 8-hour exposure limit for health (TLV) is 1000 ppm. One way to reach the LEL of a flammable solvent is by evaporation of a large amount of solvent in a process such as lacquer coating. Another is from a spill of a flammable solvent. If left to evaporate in a still air situation, concentrations of solvents vapors above the LEL can easily be achieved. Many solvent vapors are heavier than air and their vapors can collect in low lying areas.
With ventilation, the solvent vapors will not fall to floor level but will go wherever the air currents are going since the solvent/air mixture will have the same effective density as that of the air. For fire safety, you want to keep the vapor concentration in air to less than 1/4 of the LEL. (Of course, for health reasons, you want to keep it a lot lower than that). When exhausting flammable vapors, the exhaust fan must be explosion-proof if the concentration of solvent vapors in contact with the fan could exceed the LEL. Ideally, you want the fan to be as close to the source of solvent vapors as possible to prevent them from spreading throughout the area. The question is must the fan be explosion-proof?
NFPA Electrical Safety Definitions The National Fire Protection Association standard NFPA #30 and OSHA standard 1910.106, in their section on industrial plants, have certain definitions for flammable vapor/air mixtures. A Class I Division 1 location includes 5 feet in all directions from all points of vapor generation where a flammable vapor and air mixture could exist under normal conditions. A Class I Division 2 location is: 1) a location where flammable vapor/air mixtures could exist under abnormal conditions (e.g. a spill or leak); 2) in an area 20 feet horizontally and 3 feet vertically above floor level from Division 1 locations; or 3) within 25 feet horizontally and 3 feet vertically from a pump or other device handling flammable liquids.
Electrical wiring and equipment within Division 1 or Division 2 locations must meet National Electrical Code requirements for that location.
Local Exhaust Systems Inside an enclosed hood or duct, evaporation of even small amounts of flammable solvents might approach the LEL. And this could even be more of a problem in case of a spill. Therefore, electrical wiring and equipment for local exhaust systems like spray booths, enclosed hoods, and slot hoods that will be used for flammable solvents should meet Class I Division 1 standards set by the National Electrical Code - the blades should be spark-proof, the fan belt enclosed, and the fan motor outside the duct.
Dilution Ventilation According to the NFPA and OSHA standards for industrial plants, a fan less than 5 feet away from a large amount of evaporating solvent, for example, would have to meet Division 1 requirements, but not a fan more than 5 feet above the evaporating solvent. If the only chance of reaching the LEL was in a spill, then electrical wiring and equipment would have to meet Division 2 requirements. Note that these requirements would not apply to combustible solvents at room temperature since they would not form a flammable vapor/air mixture (unless sprayed).
In discussion with a NFPA representative, it seems the NFPA standard was developed for large scale solvent use. In a small scale operation, with small amounts of solvents, if an exhaust fan was running before the solvent evaporation starts, and the fan is powerful enough, then the vapor concentration will not have a chance to build up to the LEL. (If the fan is turned on in the middle of the evaporation process, a fire could start if the vapor concentration has reached the LEL near the fan).
The major concern would be a large spill where you might not have an exhaust fan running. In that case, do not turn on a fan unless it is explosion-proof. Turn off any flames, evacuate the area, and shut off the power to the room at the circuit breaker so there is no spark generated. Call the fire department for spill cleanup (or your local HazMat team).
Fire Safety Recommendations • Use combustible solvents rather than flammable or extremely flammable solvents when possible. • Use the smallest container size practical to minimize the size of spills. • Cover solvent containers to reduce chance of a spill. • Remove sources of ignition. • Have adequate ventilation for fire and health reasons. • If in doubt, get expert advice on fire safety requirements.
Checking the Ventilation System Once a ventilation system is installed, or if you have an already existing one, the crucial question is does it work? If the ventilation system was designed and installed by a ventilation engineer, air flow measurements should be taken to ensure that design specifications are met. In addition, smoke tube observations should be made to ensure that toxic contaminants are being captured adequately. This should also be done periodically to ensure that the system continues to work properly.
However, it does not take a trained engineer to do some simple tests to see if your ventilation system is working. There are some common sense rules and techniques you can use to test the effectiveness of your ventilation system.
Local Exhaust Systems A local exhaust system, for example a spray booth or slot hood, should capture the toxic contaminants before they get into the air you breathe. If you can see dusts or mists in the air or settling on surfaces, that is an indication that a local exhaust hood is not working properly. If you can smell gases or vapors, that is another indication, although do not rely on your sense of smell alone since many chemicals do not have detectible odors at hazardous levels.
You can use commercially available smoke tubes to generate a visible smoke so you can actually see the air flow into an exhaust hood. A less expensive, but still valid way to check air flow, is to use a child's soap bubble kit. If the soap bubble just slowly drift to the floor, this indicates poor air flow. The soap bubbles (or smoke) should be steadily drawn into the hood. In particular, you should always make sure that the soap bubbles or smoke are not drawn past your face when in working position, since this could indicate you are being exposed to the toxic contaminants being exhausted. If the soap bubble test indicates that the local exhaust ventilation system is not working properly, then you have to try and find the problem. The following are some common types of problems with local exhaust systems.
• Is there adequate makeup air? One of the most common problems with exhaust systems is insufficient makeup air to replace the air being exhausted. The resulting decrease in air pressure affects the performance of the exhaust system. • Is the makeup air source properly positioned? If the makeup air source is located too close to a local exhaust hood, it can create turbulence and interfere with proper functioning of the local exhaust system, and might even blow the contaminants into your face. Soap bubbles will show if there is turbulence. • Are there cross-currents? Local exhaust hoods are often very sensitive to cross-currents caused by heavy traffic around the hood, nearby air conditioners, other exhaust systems, etc. Use soap bubbles to see if this is occurring. • Is the process enclosed adequately? The more enclosed a process is, the more effective is the hood. • Are there obstructions or holes in the ducts? With dust collectors and woodworking machines, one common problem is that the duct is not properly connected to the machine, allowing air to enter the duct at the connection, which in turn can interfere with proper functioning. • Do the ducts have a lot of bends and sharp elbows? The more elbows in a duct, the greater the loss of efficiency. This is similar to the effect on water flow of crimping a garden hose. • Have the filters in spray booths been checked? Clogged filters or ripped filters will interfere with the operation of the spray booth. Installation of a manometer (obtainable from a spraybooth manufacturer) across the filter can tell you when to change filters. • Are the fans connected up properly? Sometimes, fans were connected in reverse, or even so incorrectly that the fan motor was not connected to the power switch. Connecting a centrifugal fan incorrectly will cause a decrease in air capacity. • Has the fan been checked? Worn or slipping fan belts, burnt-out fan motors, damaged electrical connections are all reasons why ventilation systems stop working. • Have there been additions to the ventilation system? A common problem with older ventilation systems is that additional hoods and ductwork have been connected to an existing ventilation system without determining whether a stronger fan is needed. • Is the exhaust air being recirculated? Recirculation of exhaust air means that the toxic contaminants are also being recirculated.
Dilution Ventilation Dilution ventilation systems bring in clean, makeup air that mixes with and dilutes contaminants to a safe concentration, then exhaust these to the outside. The following are some problems that occur with dilution ventilation systems. (See Figure 1). • Is there adequate makeup air? As with local exhaust systems, there must be adequate makeup air to replace the air being exhausted. • Is the makeup air source positioned properly? If the makeup air source is located too close to the exhaust outlet of a dilution ventilation system, the makeup air can short circuit directly to the exhaust duct and not mix properly with the contaminated air in the room. • Are the contaminants being drawn past your face? Clean air should enter the room, pass your face, get contaminated, and then be exhausted. Soap bubble observations can show air flow direction. • Are obstructions interfering with air flow? A common problem with ventilation for oil painting is that vertical easels can block the free flow of air. Carefully position makeup air source and exhaust fans to prevent this. • Is the fan connected properly? Connecting propeller fans in the wrong direction will cause air to blow into the room instead of being exhausted. This is easily detected. • Is the exhaust air being recirculated? Recirculation of exhaust air means that the toxic contaminants are also being recirculated. Using these simple rules, you should be able to see if your ventilation systems are working properly. For further information on ventilation, see CSA's book Ventilation.
1. Clark, N., Cutter, T., and McGrane, J., Ventilation, (1984). Lyons and Burford Publishers, New York, NY.
2. Committee on Industrial Ventilation. (1992). Industrial Ventilation: A Manual of Recommended Practice. 21st ed., American Conference of Governmental Industrial Hygienists, East Lansing, MI.
3. McCann, M. (1992). Artist Beware. 2nd ed., Lyons and Burford, Publishers, New York, NY