Painting is usually the final finishing operation on a product; the painted coating must be blemish-free of dust or other airborne particulates. There are two aspects of air cleaning in most ventilation systems for finishing operations, namely supply air and exhaust air. Supply air for spray booths should exceed exhausted air to provide directional air flow out of the containment area. Particles 10 µm and larger can affect paint finishes.

Most spray booths are one of two airflow designs; crossdraft or downdraft. Crossdraft booths use intake filters located at the access doors. Typically these filters have a 20-30 percent ASHRAE rating.

The make-up air supply for downdraft systems may actually first penetrate a prefilter (25-30% ASHRAE rating) before the air supply is filtered through 90-95 percent ASHRAE final filters. Since these air handling systems can harbor particulates in ductwork, media is also installed at the point that the clean air enters the booth. This media is usually made from synthetic fibers formed into a mat, and made fire retardant. It may also be treated with a tackifying agent, and is often installed as rolled goods for continuous coverage of the spray booth ceiling.

The second consideration for filtration is spray booths is exhaust air which removes the overspray. The removal of overspray can be accomplished using water wash devices or media filters. These are specially made filters which are designed to remove overspray without plugging rapidly. Depending on the chemical components of the collected overspray, special disposal techniques may be necessary to eliminate spontaneous combustion or other fire hazards.

Crossdraft intake filters are usually made with some type of polyester and supported with a wire structure. These intake filters may also be manufactured to meet high efficiency or fine filtration standards.

A cleanroom paint line is an essential part of the Budd Company's $24 million plant in Kendallville, IN. The 205,000-square-foot plant is producing components for General Motors all-purpose van and the Yamaha Wave Runner jet boat. Because the parts are visible exterior surfaces on the product, they must have a perfect showroom appearance. It was for this reason that a cleanroom environment was selected for the paint line.

The painting area is designed to eliminate dust and lint from parts being painted, cured and cooled. This is achieved by a cleanroom that isolates all painting operations from the rest of the plant. The cleanroom is in two parts: an enclosed "tunnel" for all paint processing. Parts being painted, flashed, cured and cooled never leave a cleanroom atmosphere until they enter the shipping area.

The overall painting area is a separate "enclave" from the rest of the plant. The only access into the enclosure is through the conveyorized spray washer entry, the painted product conveyorized exit and personnel access doors. Filtered makeup air from outside the plant is ducted into the enclosure, keeping it at a slightly positive pressure. This ensures that air flow through access openings is always out of the enclosure. This is necessary because SMC secondary operations are inherently dusty.

The tunnel concept further encloses finishing processes from the first spray booth until the exit from the postbake cooldown zone. Air makeup for the "tunneled" area is drawn from the already filtered air in the painting enclosure. This air is filtered again entering the tunnel. The air makeup keeps tunnel air pressure slightly positive in respect to air in the rest of the painting area to ensure that only double filtered air enters the tunnel. Makeup air for each spray booth is filtered separately down to 3 microns.

The Chrysler Corp.'s St. Louis, MO assembly plant is one of 11 plants in North America where two electrostatic color-painting tunnels are connected via a common contamination-controlled vestibule. Positive pressure is maintained from the paint shop to the vestibule and from the vestibule to the rest of the plant. To enter the paint tunnel, air flows through multiple levels of air filters including high-efficiency bag filters and exits via panel-filter/diffusers in the paint tunnel ceiling. The maximum allowable particle size is 3-5 microns. The air flow is like a conventional cleanroom from the ceiling through a grated floor. The velocity is also similar with 100 feet/minute in some zones and 60-80 feet/minute in other zones.

The powder coating booth is maintained at a positive pressure with filtered air. Personnel entering the booth must wear protective garb, including boots, to minimize carry-in contamination. Before entering the powder booth, the bodies move through a deionized-air blow-off ring. This is designed to remove electrostatically-attracted lint.

The entire paint line is in a cleanroom environment, with filtered air and entry/exit blow-off vestibules for personnel. In addition, makeup air for all spray booths is doubly filtered. Compressed air supplying spray guns and bells is also doubly filtered for cleanliness. The paint line operates its own "dirt library" to help control contaminants. Paint-surface defects are microphotographed. The photos are then analyzed and filed by contamination category.

Operators are warned against using hand lotions that may contain potential contaminants, such as silicones. The intense efforts for cleanliness helps minimize the need for sanding, which is another potential source of contaminants.

When paint finishing is done by human beings, additional precautions must be taken to protect the worker and reduce contamination from the worker's presence in the chamber. When addressing respiratory protection for workers, manufacturers must comply with specific health and safety regulations. Both finishers and coatings manufacturers must be familiar with OSHA (Occupational Safety & Health Administration) general industry standard (29 CFR 1910.134), which includes 11 requirements for a minimally acceptable respiratory protection program. This standard has been redesignated as 29CFR1910.139 Respiratory Protection for M Tuberculosis and will continue to apply to respirator use for protection against TB exposure until OSHA finalizes a specific TB standard.

There is no "all purpose" respirator that will provide protection against all contaminants and concentrations in all workplace environments. Employers must match expected respiratory protection needs with the respirator models available.

Once a company has identified the type of airborne contaminant and its concentration, it can select the most appropriate style of respirator. Industrial respirators can be categorized into two major groups: positive pressure and negative pressure.

Positive-pressure respirators provide a flow of clean breathing air to the user from a compressor, compressed-air cylinders, portable pump or powered air purifier. Positive- pressure systems may consist of helmets, hoods, full-face or half-mask respirators.

Positive-pressure systems are used primarily when concentrations of contaminants are greater than 10 times the Personal Exposure Limitation (PEL), when half-mask air purifying respirators are not allowed. (For example, the OSHA standard for abrasive blasting requires an air-supplied respirator only.)

In addition to supplying respiratory protection, positive-pressure systems may be desirable for other reasons, such as additional protection and worker comfort.

OSHA has specific standards (such as lead and cadmium) requiring that a powered air purifying respirator (PAPR) with high-efficiency filters be provided if requested by the worker. Due to the continuous flow of air during both inhalation and exhalation, there is no resistance to breathing, which may encourage longer wear time.

Negative-pressure respirators help remove airborne contaminants as the worker breathes by drawing ambient air through filters or cartridges. These respirators are tight-fitting and available in quarter-, half-and full-face models. The advantages and disadvantages of each group must be considered in the respirator selection process.

Negative-pressure systems require no mechanized airflow other than that of a worker, who breathes air through filters or cartridges to purify it. Chemical cartridges and filters are available for a number of contaminants.

Negative-pressure respirators can be categorized into two types: maintenance-free particulate respirators and elastomeric respirators. Both types must meet or exceed National Institute of Occupational Safety and Health (NIOSH) requirements for service life, breathing resistance, filter efficiency and quality assurance. Elastomeric facepieces can be further classified into three types: maintenance-free, low-maintenance and conventional-maintenance.

The National Institute of Occupational Safety and Health (NIOSH) is the government agency responsible for evaluating respirators' performance, classifying their use and approving their effectiveness. Since July 10, 1996, respirators are certified under a new public health standard, 42 CFR Part 84. This new rule revises test requirements for particulate filter respirators and prefilters for gas and vapor respirators, such as those for spray paint.

Under a former approval scheme, maintenance-free half-face particulate respirators (sometimes referred to as disposable dust respirators) and prefilters were approved for use based on the type of contaminant, such as dust/mist, dust/fume/mist, and high-efficiency particulate air (HEPA). Under the new 42 CFR 84 standard, they will be classified as either N, R or P series filters (depending whether they are "not oil resistant," "oil resistant" or "oil proof") with three levels of filtering efficiency: 95 percent, 99 percent and 99.97 percent. The elastomeric face piece respirators carry a variety of approvals depending on how they are equipped with cartridges or filters.

One of the biggest challenges involves determining which class of filter to use. The N series can be used in work settings free of oil, since some oils eventually degrade performance of this filter type. However, R and P series filters may be used in work environments where there are oils. Because this industry deals with so many different contaminants (including resins, adhesives and inks), each situation or operation must be evaluated and the proper filter chosen for the conditions.

Essentially, to be cost effective, the N, R and P series filters should be tested with a variety of "oily" chemicals. If the filtration effectiveness of the N series is degraded, the more costly R or P series may be needed.

OSHA's respiratory protection standards and their subsequent revisions cover an estimated 5 million respirator wearers working in an estimated 1.3 million workplaces in the covered sectors.

Metal finishing operations may require similar ventilation, exhaust and personal protection systems. For example, mechanical ventilation minimum rates are 2000 cubic feet per minute per welder. Local exhaust hoods or ventilating booths are used to move air at fixed stations, and depending on work performed, such as portable grinding operations, should be conducted within a partial enclosure. The opening should be no larger than is actually required in the operation and an average face air velocity of not less than 200 feet per minute must be maintained. Individual filter requirements are dependent upon the work being done, the air flow provided, and whether personal protective gear is used by workers. Other safety issues regarding eye, limb, or other protection are not addressed in this work; nor are operational safety issues regarding explosion proof motors or other potential hazards. Additional information for U.S. requirements are available from the U.S. Department of Labor, OSHA, NIOSH (a division of Center for Disease Control) and comparable agencies at a state and local level.