Tiny Particles, Big Danger: Technological Advances in Identifying Airborne Health Risks

By Chad Born, BSc, MBA, CIH, Director, Industrial Hygiene and Microbiology

2000 years ago, Gaius Plinius Secundus, better known as Pliny the Elder, found refiners using animal bladders in an effort to limit inhalation of dust including lead particulates. Workers covered their noses and mouths with the bladders to remove some of the harmful particulates from the air they were breathing in¹. Today we have dramatically improved our respiratory protective equipment and have also made significant advancements in particulate identification and hazard classification. Particulate Threshold Limit Values (TLVs) have been established for every workplace and are designed to protect employees from dangerous airborne substances.

General Particulate and Human Exposure Information

Particles are small masses of solid or liquid matter. The hygienist’s interest in particulates revolves around knowledge that there are a great number of materials which, when aerosolized, are either hazardous to one’s health or cause irritation. The hazard potential of a material depends upon its concentration and the area where it is deposited in the body. Aerosolized particles typically range in diameter from 0.01 µm to 1,000 µm with most particulates somewhere in the range of 0.1 µm to 100 µm in diameter.

Current research implicates particulates, in specific sizes and quantities, as a cause for certain workplace injuries. As such, Threshold Limit Values were created that take particle sizes into account. Some substances actually have multiple TLV limits, e.g. glass fibres in air have both an allowable fibre count per mL of air and an inhalable particulate amount in mg/m3. Molybdenum, likewise, has both a respirable and inhalable limit. As such, consultants may declare an area compliant (with regard to TLVs) but multiple parameters & size levels were not tested and therefore the area may have to be re-sampled.

Aerodynamic Equivalent Diameter

According to the US Environmental Protection Agency, the Aerodynamic Equivalent Diameter (AED) is the diameter of a unit density sphere having the same terminal settling velocity as the particle in question, whatever its size, shape and density. Essentially, it standardizes all particles into a spherical shape and a given unit of density. When referring to a 4 µm particulate we are referring to a particulate that is 4 micrometer aerodynamic equivalent diameters.

AED measurements are used to predict where in the respiratory tract such particles may be deposited.

The 50% Cut Point

When using a cyclone sampler, the centrifugal force causes solids larger than the cut point size to be thrown to the inner wall of the device (much like a centrifuge). These larger particles fall down the inner wall to a collection point for later discharge. Smaller particle (ones of interest) are drawn up and trapped on a filter for analysis. As can be seen in the respirable particulate table below, (respirable particulates have a 50% cut point of 4 µm) 50% of the 4 µm AED particles are collected.

Particles tend to be deposited in different areas of the pulmonary system depending on their size. Smaller particles tend to have lower allowable limits for exposure. This is in part due to the fact that they reach further into the pulmonary system, often to places when our body’s defence mechanisms – coughing, ciliated cells and warm moist air (which causes particulates to take on weight and fall out of suspension early on), etc. many not be able to defend against them. And if these smaller particles are also inorganic or insoluble, the white blood cells (one of the last remaining defences if the particulates reach far enough into the pulmonary system) may have a hard time removing them. Therefore, these small particulates have the lowest allowable TLVs. However, certain ultra-small sized particulates (< 0.001 µm), behave more like gases than particulates. At this stage, particulates can actually be breathed in and out without deposition onto lung tissue.

Analytical Techniques

Analytical Separation

Analytical Separation, also known as “dust counting”, was first introduced in the 1920’s. It involves drawing air through an impinger, trapping particulates in the impinger’s liquid and microscopically counting the particles based on specific size criteria. This method was recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) until 1984. Results were typically reported in millions of particles per cubic meter of air. From a laboratory analyst’s perspective, the time and effort involved in counting millions of microscopic particles every day would have been incredible. Interestingly, fiber in air counts (asbestos and others) are still performed based on size selective microscopic counting techniques following NIOSH 7400 “A” and “B” counting rules.

Mechanical Separation

Analysts, previously counting millions of tiny particles, can now determine particle size using a much simpler method called generally called Mechanical Separation. There are many size-selective sampling devices that allow reasonably specific size ranges to be determined. Inhalable samplers, cyclones and fine particulate matter samplers (PM samplers) are a few of the possibilities available to hygienists today.

There are three sizes involved in mechanically separated particulates and most involve gravimetric weighing as part of their analysis. Laboratories generally pre-weigh filters with specific pore sizes for the required analysis. This involves removing all electrical charges and controlling humidity, temperature and air movement at each weighing session. The gravimetric work includes laboratory filter controls, lab blanks, field blanks, duplicates, reference weights, humidity and temperature control and deionization checks. This level of weighing typically involves very knowledgeable scientists interested in producing the highest quality results.

 Respirable particulates (the smallest of the three discussed here) have a 50% cut point at 4 µm. They are sampled using a cyclone which causes the air to be spun and utilizes centrifugal forces to effect particle separation. These particles can penetrate to the gas exchange region of the lungs and typically have the lowest allowable levels.

Thoracic dust, also known as PM10 dust, has a 50% cut point at 10 µm. They can be sampled using a number of different size selective impactors. These particulates can penetrate into the gas exchange regions and thoracic regions.

Inhalable dust, new to many hygienists, is dust which has a 50% cut point at 100 µm. Inhalable dust is most closely related to total dust but don’t confuse the two. They are not the same. Total dust is described as dust which is collected by simply drawing air through a 37 mm pre-weighed filter at a flow rate between 1 and 2 L/min. With total dust, the assumption is that all dust sizes in the aerosol are collected. Inhalable dust is similar to total dust but considered to more accurately describe the particle sizes of concern due to a set cut point and ability to collect and retain all particulates collected during sampling in normal work environments. Therefore, it is not acceptable to use a sample for total dust as justification for meeting an inhalable dust limit. In fact, there is data to suggest that inhalable dust sampling, even though it limits the size of particulates collected (particles larger than 100 µm are limited), actually collects more dust due to the significant losses experienced during total particulate sampling in typical workplace conditions.

As always, more information is needed. We have improved our understanding of aerosols, particulate sizing and the associated risks to human health. Currently, there is a good understanding of factors such as particulate shape, density, solubility and size which help us predict which particulates reach, and potentially damage, different areas of the pulmonary system. Analytical methods of sampling and analysis have improved significantly over the last century and workplace threshold limits are reflecting the information coming forward.

Chad Born is the head of CANTEST’s Industrial Hygiene and Microbiology group. He can be contacted at cborn@cantest.com or 1 800 665 8566.

¹Patty, F.A.: Industrial hygiene: retrospect and prospect. In Patty’s Industrial Hygiene and Toxicology, vol. 1, 3rd ed. New York: John Wiley and Sons, 1978.

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