Airborne Dangers

May 1, 1999
Knowing the facts about airborne particulates can make you more effective in managing the hazards they present.

Industrial hygiene is the practice of identifying and controlling health risks in the workplace. This article will discuss particulates in air, what they are, how they are formed, what makes them breathable and what the health and safety professional can do to control risks associated with their presence. Specifically, it addresses how aerosol characteristics affect health hazards and risks and explains their influence on your decision regarding the selection, design and implementation of risk control strategies.

What Is an Aerosol?

Suspensions of particulates in air are called aerosols. These suspensions may be either solid or liquid, and examples of both of these are found throughout nature. Pollen or ice in the air are examples of solid, naturally occurring aerosols. Fog and mist are examples of liquid aerosols.

Aerosols are generated by any number of industrial processes, usually in the forms of fumes, dusts and mists and all with particulates small enough to be inhaled.

As always, the first question to ask about an aerosol or any airborne contaminant is "What is it?". The answer to this question leads us to the first step in controlling the risk. Once we understand the process and how it releases contaminants, we can offer guidance on how to control the risks associated with both the process and the material produced.

Aerosol Types

Fumes are produced from vaporized solids that recondense into very small, solid particles. Soldering, welding, torch cutting and general milling operations generate fumes. A portion of diesel exhaust is also a fume. Because fume particles are so small, they can be inhaled deep into the lungs, then easily absorbed into the bloodstream.

Dusts are solid aerosols formed by mechanical means. Grinding, sweeping, shot blasting, sanding and using compressed air to clean surfaces are examples of processes that generate dusts.

Mists are liquid aerosols formed by mechanical means. Spray painting, power washing and even the action of waves on the shore form mists. Mists are complex, in that they often contain a solid nucleus. If this nucleus were, for example, a pigment, it would present particular health hazards and require control techniques that are different from those involved, were there no nucleus present.

Size Matters

Aerosols range widely in size. Fumes can be as small as 0.01 microns, whereas dusts and mists can range up to 100 microns and more. The significance of particle size is found in two related concepts.

The first is that large particles tend to settle out of the air more rapidly than do small ones. The settling rate for sub-micron particles is so slow as to be inconsequential. These particles stay suspended in air and drift with ambient air currents. A 0.01 microns particle will sink through air at a rate of about 140 days to settle1 meter in air. A 0.1 microns particle will settle about 10 times as fast, and will require about 14 days to settle 1 meter. A 1 micron particle will require about one hour to settle 1 meter. The point is that these small particles remain in the air long enough to be inhaled, and they will remain in the air long enough to be swept around by ambient air currents.

Even a process of relatively short duration is capable of generating a respirable aerosol that can stay in the breathing zone all day. Larger particles (between 5 and 10 microns and larger) will settle out of the air more quickly, leaving little opportunity for workers to breathe them over time.

The second reason particle size is important is that the big particles get trapped on the way down into the lungs, while smaller particles are drawn deeper into the lungs. Larger particles have enough mass and inertia that they will be trapped in the airway between the nostril and bronchi when inhaled. Particularly large particles become trapped in the nose, and are expelled by sneezing or blowing the nose. As the air stream turns at a split in the windpipe (i.e., bifurcation of the bronchus), larger particles become trapped in the airway. Smaller particles that are trapped in the bronchial region will be expelled via the mucocilliary escalator.

Depending on where we are, whom we are with, and whether or not we are wearing respirators, these airborne contaminants will either be swallowed or spit out.

The American Conference of Governmental Industrial Hygienists has embraced a relatively new concept in measuring aerosols. Rather than the traditional "respirable" or "total" fractions, the ACGIH now discusses three fractions: inhalable, thoracic and respirable.

Inhalable particulates are those that can enter the airway particulates up to 100 microns in size. Inhalable particles are considered when they can affect health, regardless of their size. Lead aerosol is an example of this.

Even though the fume-sized particles can enter the body through the lungs, the larger particles will be swallowed. This is true for most aerosols that can cause a systemic ill health effect: they can be absorbed by the respiratory or gastrointestinal tracts, so particle size in unimportant.

Corrosive aerosols are treated the same way, but for a slightly different reason. They cause irritation or tissue damage, no matter where they make contact with the respiratory tract.

Thoracic particulates are generally 10 microns in size or smaller. These aerosols are deposited in the bronchial region of the lungs. Sampling for these usually is done when there are concerns for this region as a target organ.

Cadmium, diquat, nickel and continuous filament glass fibers are contaminants which present a health risk from contact in this area of the lungs.

Respirable particulates are generally smaller than 4 microns. They cause health effects deep in the lung; however, only a size-selective sampler would be used if the deep lung is the only target organ. The best example of a respirable particulate is crystalline silica. Though a recognized cause of fibrotic lung disease (i.e., silicosis) and cancer, crystalline silica appears to cause no ill health effects when ingested.

What Should Be Done?

Once we understand the material and the hazards it poses, we look at the risks associated with the process. We want to learn about particle size (if relevant), how the aerosol is released and how it can be controlled.

If size-selective sampling is needed, use either the traditional cyclone samplers (such as those sold by SKC, MSA, or other distributors) or other size-selective samplers. The Respicon, sold by TSI, is a three-stage unit that samples all three fractions. The IOM sampler, sold by SKC, samples the largest, or inhalable, fraction, and the Spiral Sampler, sold by SKC, samples for the smallest, or respirable fraction. Any of these tools will provide useful information. The real question is: What to do with the information we get from sampling?

Using Engineering Controls

Next to replacing the hazardous material with nonhazardous product, engineering controls are considered the best way to control the risk of ill health effects in the workplace. Look at engineering controls whenever you need to control the release of a material to the workplace. The basic concept is housing the process in a conceptual box that is as small as possible. The larger the box, the more energy is required to control the particles.

Three engineering controls capturing the aerosol, keeping the aerosol moving and using respiratory protection are common.

Capturing the aerosol: Designing and engineering the control system is not always a straightforward process. The shape of the enclosure or exhaust vent is always an important consideration because the "box" must be as efficient as possible. Air velocity within the ductwork is of critical importance. The Industrial Ventilation Manual, published by the ACGIH, is probably the best resource available to address this issue.

The composition of the aerosol is also important. Some aerosols are quite sticky, and accumulate on any surface they touch. Because of this, filtration at the point of collection is needed so the collection system itself remains in good working order.

Plastic fume and paint spray are two examples of sticky aerosols. Plastic fume, which is very small, and additives, such as ultraviolet stabilizers or flame retardants, accumulate just inside capture openings as the air begins to cool.

The buildup of plastic and other chemicals rapidly decreases the performance of the system. The solution is often to place a filter just inside the collection duct, but the filter must be changed and kept in place.

Paint booths generally have some sort of filter bank as part of the booth. The absence or malfunction of an adequate filtering system will likely lead to the destruction of the fan itself. The impeller becomes so clogged with paint buildup that it is easier and cheaper to replace it than to clean it. This is especially true of urethane coating systems.

Keeping the aerosol moving: Minimum flow rates for aerosols are much higher than minimum flow rates for gases or vapors. The larger the particle size, the higher the required air flow. This is because the aerosols must not be allowed to settle out of the air stream while in the duct work. This settling in the duct work is similar to arteriosclerosis. Once is starts, it progressively worsens, the endpoint being a situation in which local exhaust ventilation is ineffective.

Respiratory protection: Respiratory protection is often used where local exhaust ventilation is not present or does not achieve the desired goals.

The type of aerosol is of particular importance for respiratory protection. Dusts and fumes can generally be controlled with an N-class filter. Liquid aerosols will require an N-, R-, or P-class filter, depending upon whether the aerosol is an oil aerosol and how long the filter must last.

Many manufacturers of particulate filters rely on electrostatic interaction to handle the smallest particles. Oil aerosols tend to wet the filter fibers, eliminating the electrostatic charge. Manufacturers have had varying degrees of success in controlling this tendency, so careful selection of the particulate filter is necessary.

Paint spray aerosols present a unique challenge: they are comprised of two distinct portions: the liquid carrier and the solid pigment. The particulate filter is chosen for the best control of the liquid aerosol. Once captured, the carrier evaporates and the vapors move through the filter. A vapor cartridge must be put behind the particulate filter to capture the aerosol vapors as they are generated.

Clearly, it is important to understand the nature of the aerosol being generated, the type of aerosol, its chemical composition and its particle size all have an impact on health risks and to determine the best method for controlling them.

David Abrams, CIH, is a consultant with ARS Environmental Health in Minnetonka, Minn. He is a member of the American Industrial Hygiene Association"s respiratory protection committee.

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