PPP: Breathing Easy

The best way to combat unhealthy fumes is to remove them at their source, before they can contaminate indoor air.

Vented exhaust hoods capture heat, fumes and unwanted odors, and vent them directly to the outdoors or into the building’s existing ductwork or ventilation system.
The sophisticated clays, slips and glazes used by today’s potters offer enhanced properties to improve the end product, but they also contain many organic and inorganic chemicals and minerals, such as formaldehyde, that can release toxic fumes when the products are heated to high temperatures in kilns. The best way to combat this environmental problem is to remove unhealthy fumes at their source, before they can contaminate indoor air.

Using vented exhaust hoods over ceramic kilns is a proven way to accomplish this goal. These devices are typically composed of a circular metal hood, a motorized exhaust fan and a mounting kit appropriate for the application. The canopy hood, which can be raised and lowered or moved from side-to-side, captures heat, fumes and unwanted odors, and vents them directly to the outdoors or into the building’s existing ductwork or ventilation system. This eliminates the dispersal of concentrations of unhealthy fumes into confined indoor areas, while also helping to maintain comfortable working temperatures.*

Figure 1. Ventilation fans vs. vented exhaust hoods.

An Organic Problem

Sources for organic materials in clays and glazes can include deflocculants, stabilizers and thickeners (e.g., carboxy methyl cellulose, propylene glycol, gum arabic, etc.); coloring agents; preservatives such as formaldehyde or orthophenylphenol; luster glaze oils, resins and waxes; and natural organic impurities. In addition, various clay and glaze components, some of which are included in proprietary formulations, are considered to have potentially adverse health effects if inhaled. A few of these include:

• Arsenic trioxide (AsO3), a carcinogen that can cause kidney damage, numbness in hands and feet, anemia and irritate moist membranes.

• Barium (carbonate [BaCO3], chromate [BaCrO4] or oxide [BaO]), a poison that can cause spasms, contractions of the blood vessels and muscles, hypertension, heart arrhythmias and digestive disturbances.

• Cadmium (oxide [CdO] or sulfide [CdS]), which can cause severe lung irritation with fatal reactions after as little as six hours of exposure. Symptoms include a runny nose, soreness and a prickling sensation in the nose, drying of mucous, loss of sense of smell, disabling emphysema and kidney damage.

• Nickel oxide (NiO), a carcinogen that can cause “metal fume fever” (irritation of the nose, mouth and throat, cough, stomach pain, headache, nausea, vomiting, metallic taste, chills, fever, pains in the muscles and joints, thirst, bronchitis or pneumonia and a bluish tint to the skin; symptoms generally disappear within 24-48 hours and leave no effect). It can also severely irritate skin.

• Lead compounds, which are unhealthy and cumulative. Very small quantities can poison a child, and the fetus is at high risk if the mother is exposed. Symptoms can include fatigue, constipation, sleeplessness, headache, double vision, pains in the arms and legs, psychological changes, anemia, colic and neuritis.

Other emission dangers associated with kiln operation include metal fumes, sulfur oxides and carbon monoxide. Metals can begin to vaporize and give off fumes long before they reach their boiling point. Toxic metals such as lead cadmium, antimony, selenium, copper, chrome and nickel are known to volatize during firing, and nickel and chrome are considered to be carcinogens. Sulfur oxides readily combine with water to form sulfuric and sulfurous acids. These acids are released from almost all clays during bisque firing and also evolve from some glazes, and inhaling them can cause chronic bronchitis or asthma. Carbon monoxide can result from the incomplete combustion of organic materials in the kiln. Headache and fatigue are early symptoms of carbon monoxide exposure; concentrated or prolonged exposure can be harmful.

These substances and fumes are not present in every ceramic or pottery facility. However, it’s always better to err on the side of caution when considering your employees’ health. Installing a hood and exhaust fan for every kiln, and using a vented spray booth for spraying glazes and other materials has been recommended for years by various industry and safety organizations to prevent exposure to the kinds of toxins listed above. Other methods have also been used, such as using area ventilator fans or requiring employees to wear respirator masks. However, area ventilation fans remove fumes only after allowing them to contaminate the entire room or work area (see Figure 1). Respirator masks are recommended by OSHA as a secondary means of protection; during times when the primary ventilation system is being installed, maintained or repaired; or during emergency situations. However, they are not acceptable as a primary means of protecting against airborne toxic substances.

By using a vented exhaust hood above their kilns, pottery producers can maintain a comfortable work environment while meeting OSHA regulations and capturing fumes at their source.

Figure 2. Sizing vented exhaust hoods.

Sizing and Installation

Selecting the proper hood size for a vented exhaust hood system begins with measuring the area above the kiln to be vented. For cylindrical areas, including six- and eight-sided kilns, measure the diameter of the kiln and add 4 in. (see Figure 2). For example, if the kiln diameter is 40 in. in diameter, a 44-in.-diameter hood is required. If the kiln is square in shape, measure the diagonal and add 4 in. to determine the hood size. If two side-by-side kilns are always fired at different times, a swinging wall bracket can be installed so that one vented exhaust hood system services both units. In such cases, the measurements should be based on the largest diameter (or diagonal) of the two kilns.

Hoods are typically available in standard diameters from 22 to 54 in. with corresponding hood heights from 4 to 12 in. Material construction can be 0.063 aluminum, polypropylene, 18-gauge galvanized steel or 304 stainless steel. Standard models are designed for electric kilns, so companies that are using gas-fired kilns or kilns with customized or irregular hood configurations will need to contact the exhaust hood supplier to determine their size and material options.

The exhaust fan component of the vented exhaust hood system typically includes a fan and motor. Some models feature a two-speed, thermally protected motor and can handle up to 265 cubic feet per minute (CFM) at zero static pressure. Other models have the same features with a standard capacity of 500 CFM. For companies with an exhaust hose that is longer than the standard 10 ft, fan motors (blowers) also come in capacities of 800 CFM, 1000 CFM, 1200 CFM and larger. Systems with explosion-proof motors are also available for applications with volatile emissions; however, these types of systems generally aren’t required in the ceramic industry.

Figure 3. A positive vs. negative pressure system.
In standard installations, the blower is mounted on the hood to create a positive pressure system, whereby the blower pushes the fumes and heat to the outside. Mounting the blower at the point of exit through a wall, window or roof creates a negative pressure system, which pulls the unwanted fumes and heat to the outside (see Figure 3). A positive pressure system is suitable for companies that have easy access to their exhaust ductwork and can implement a regular maintenance schedule that includes inspecting the flexible duct for any damage, such as tears. A negative pressure system, which is slightly more expensive, is ideal for applications where the ductwork is hidden or more difficult to access, since the system will continue to pull the air and heat out even if there is a leak in the hose. Side curtains should be added when extreme cross drafts are present to ensure efficient operation.

A counterweight pulley assembly is used to raise and lower the canopy hood. For low-clearance installations, or applications where the ceiling height exceeds 12 ft or where the vented exhaust hood system will service two side-by-side stations, a swinging wall bracket is used instead (see Figure 4). The ability to swing the hood to the side allows kiln operators to more easily load top-loading kilns, especially when overhead space does not allow the system’s hood to be raised sufficiently. Stationary hoods are advised for front-loading kilns.

Vented exhaust hoods do not impede kiln operation and do not require any modifications to existing kilns or to the building, other than ventilation access. If the location of a kiln is changed, the exhaust hood can be disassembled and reinstalled where needed.

Figure 4. Schematic of a swinging wall-mounted system.

Environmental Benefits

In addition to capturing and venting dangerous fumes, vented exhaust hoods offer other environmentally beneficial performance characteristics. For example, compared to other types of venting systems, exhaust hoods allow for the use of smaller, less expensive hoods and lower capacity exhaust fans, thus lowering energy costs by permitting the use of lower horsepower motors. Since excess heat is also eliminated, workers enjoy more comfortable conditions year-round. In the summer months, these systems can save energy and costs by reducing the requirements for air conditioning.

The most significant benefit, however, is the exhaust hood’s fume removal capabilities. By capturing and venting fumes at their source, vented exhaust hood systems eliminate the environmental problems and potential health risks of contaminated air from the operation of kilns.


*Once vented to the outside, the fumes from kilns disperse so widely into the atmosphere that OSHA regulations (such as 29 CFR 1910.1048 covering formaldehyde) typically are not applicable.

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