PART I: The Science of Exposure Risk

The hiss of abrasive blasting is a familiar sound in shipyards, construction sites and industrial facilities worldwide. However, amidst this din of productivity, an important question often goes either unasked or without a satisfactory answer: How can workers and employers truly know if their blasting environment is safe? As a manufacturer of blasting abrasive products, our company is challenged with the same question. Are our abrasives safe? We’ve researched the toxicology of abrasives extensively because our customers deserve the best answer that we can provide.

Fig. 1. Abrasive blast-cleaning carries inherent risks, including long-term health risks tied to exposure to abrasive dusts. Photo: courtesy of the author

Abrasive blasting, while essential in many industries, carries inherent risks that extend beyond the immediate physical hazards of high-pressure equipment, high-speed abrasive jets, and working at heights or in enclosed spaces. The abrasive materials used to clean, profile and prepare surfaces can themselves pose long-term health risks to workers. This is why it is important to understand the science behind toxicity assessments and how they can and should be used to inform our decision-making.

In this article, we will delve into the science of abrasive toxicity, exploring the fundamental principles that govern toxicity assessments, the regulatory frameworks shaping industry practices, the toxicology research that has been published, and how it all can be used to make decisions that will ultimately keep the workers we all care about as free from harm as possible.

The Fundamentals of Toxicity Assessment

The science of toxicity assessment forms the foundation upon which all safety standards, regulations, and best practices are built. To grasp the complexities of properly defining and measuring exposure risk in abrasive blasting, it is first necessary to understand the fundamental principles of how the response of biological systems to various levels of a substance over different time periods is studied and quantified.

Fig. 2. Each abrasive material type has a unique and often complex material composition and exposure risk profile. photo: funtay / Getty Images

Perhaps the most important concept in toxicology is the dose-response relationship, first articulated by Paracelsus in the 16th century, which posits that “the dose makes the poison.” This means any substance can be harmful if the exposure is high enough, and conversely, even traditionally “toxic” substances may have no adverse effects at sufficiently low doses. This principle is particularly relevant in abrasive blasting, where the concentration of airborne particles and the duration of exposure can vary widely based on the materials being used and the specifics of the application.

Dose-response relationships are established through carefully controlled studies, often beginning with laboratory studies and animal exposure testing. Human epidemiological studies that document the health outcomes of workers exposed to specific substances at various levels over varying time frames are also extremely valuable, although they are expensive and often require years or decades to complete. It can also be quite difficult to isolate the effects of the substance being evaluated from human epidemiological data. These foundational studies are performed by scientists in a wide variety of organizations, including universities, government institutes, private laboratories, and industry research associations, among many others.

The aim of these exposure studies is to identify the threshold dose at which a substance begins to produce adverse effects, as well as the nature and severity of those effects at different exposure levels. For abrasives, this might involve examining lung function changes, cellular damage, or long-term lung health outcomes in animals or in human populations with varying levels of occupational exposure to inhalable and respirable abrasive dusts.

Understanding these fundamentals of toxicity testing moves us well along the path toward answering the question posed at the beginning of this article. As you will see, it is this fundamental exposure data that provides the best answer to whether an abrasive material or blasting work environment is causing health risks for workers.

How the Science Drives Regulatory Frameworks

In the United States, the Department of Labor’s Occupational Safety and Health Administration (OSHA) plays a pivotal role in setting and enforcing standards that directly impact the abrasive blasting industry, primarily through the Hazard Communication Standard and the Toxic and Hazardous Substances Standards. The Hazard Communication Standard (HCS), codified in 29 CFR 1910.1200, is often referred to as the “Right to Know” law. It ensures that information about the hazards of substances in the workplace is communicated to workers and is based on available scientific evidence, including especially the laboratory, animal, and human studies discussed previously. The HCS bridges the gap between scientific knowledge and practical workplace safety.

As a general rule, the HCS requires employers to classify hazards based on toxicological exposure data for the substance or mixture as a whole if such data is available. If it is not, then available exposure data for individual components of the substance or mixture must be considered. A key exception to this primacy of exposure data for a mixture over data for individual ingredients is made for classification as to the carcinogenicity of a substance in humans. If a component chemical or metal in a substance or mixture is a known carcinogen, then that component must be considered in classification of a mixture for carcinogenicity.

The question then becomes, how is the carcinogenicity of a substance or mixture established? Who reviews the exposure study results and makes the decision about whether the substance, mixture, or component of a mixture is carcinogenic in humans? That role is filled by authoritative bodies around the world like the U.S. National Toxicology Program (NTP), the American Conference of Governmental Industrial Hygienists (ACGIH), and the World Health Organization’s International Agency for Research on Cancer (IARC). Regulatory agencies like OSHA use the data, lists of carcinogens, publications, and conclusions of these organizations to set rules and standards concerning the communication of hazards and allowable exposure levels in the workplace.

Fig. 3. Some exposure to abrasive dust in a blasting workspace is inevitable and the toxic effects depend on the chemistry of the material, dust levels and the duration of exposure. Human, animal and laboratory studies of material dusts themselves provide the best assessment of the risks to workers. Photo: courtesy of the author

The conclusions of these organizations are critically important. They form the basis for regulations and policy worldwide, including in the United States. The experts at these agencies understand how to interpret the foundational exposure data and dose-response relationships produced by scientists all over the world. Their working groups of trained and experienced scientists review the studies and are able to draw sound conclusions based on the best available data. In fact, Section 6.4 of Appendix A of the OSHA Hazard Communication Standard states that when classifying for carcinogenicity, the conclusions of NTP and IARC can be considered as having established the carcinogenicity of a substance.

A major challenge in the abrasives industry is that blasting materials are complex mixtures containing trace quantities of a variety of component metals, some of which are listed by NTP and IARC as human carcinogens. Does the presence of those trace metals in the workplace, particularly in the dust floating in the air, create an unsafe environment? Is there a way to know for sure?

The Complexity of Abrasive Materials

When considering worker safety while using abrasive materials, it is important to recognize that we are rarely dealing with simple, single-component substances. Most abrasives used in industrial applications are comprised of various mineral oxides, metals, and other compounds. This complexity presents unique challenges in assessing and minimizing exposure risk.

Abrasives can be broadly categorized into natural, synthetic and recycled materials. Natural abrasives mined directly from the earth include silica sand, garnet, and staurolite. Aluminum oxide, silicon carbide, ceramics, and steel abrasives are considered manufactured or synthetic abrasives, although steel is often recycled as well. Recycled abrasives, increasingly popular for their sustainability benefits, include materials like coal slag, copper slag, nickel slag, crushed glass, and proprietary superoxalloy abrasives. Each of these categories, and indeed each specific type of abrasive within them, has a unique chemical composition and risk profile.

Fig. 4. Personal protective equipment can limit exposure to harmful contaminants, but workplace monitoring should also be employed. photo: courtesy of the author

The challenge lies in the fact that the toxicological exposure risk of a complex mixture is not necessarily driven by the presence of trace hazardous ingredients. Interactions between components and the behavior of the material as a whole can alter toxicity in ways that are not always predictable from studying the individual constituents. For instance, one component might enhance or inhibit the bioavailability of another, potentially increasing or decreasing a constituent’s toxic effects. Some constituent combinations in an abrasive might result in reduced or increased overall toxicity compared to individual components by virtue of the material’s unique chemistry. Further, exposure is not solely determined by the composition of the material. Dose, or dust level, along with exposure duration matter as well. Some abrasives produce more dust during blasting than others.

Moreover, the physical properties of the abrasive mixture—such as particle size distribution, shape, surface characteristics, and immunological clearance rate in living organisms—can significantly influence its potential health impacts. These properties affect how the material behaves when airborne, how deeply it can penetrate the respiratory system, its duration in the body, and how it interacts with biological tissues.

Toxicology is an immensely complex subject that cannot and should not be reduced to an elemental analysis of a material to measure trace quantities of listed metals and other substances. The results of such an assessment do little to properly inform employers or exposed workers who are concerned and uncertain as they face these exposure risks on a daily basis. The risks that worry them are real and the decisions they make have real health consequences.

Exposure Testing: The Gold Standard

Assessing the exposure risk of abrasives by merely examining their individual components is insufficient. The importance of whole-mixture exposure testing cannot be overstated. It allows for an evaluation of the combined effects of all components, including the effects of trace hazardous constituents and any synergistic or antagonistic interactions. It also accounts for the physical characteristics of the abrasive as they are actually present in a workplace, providing a more accurate picture of potential exposure risks.

This has long been well understood in the world of toxicology. Back in the 1980s and 1990s, silica sand was widely used as a blasting abrasive. There were growing concerns about exposure to the crystalline forms of silicon dioxide (quartz, cristobalite, and tridymite) in inhalable and respirable silica sand dusts, which were known to persist in the lungs and cause lung silicosis and sometimes cancer. There are documented cases of workers who ultimately died from the effects of long-term exposure to the dust from silica sand.

Fig. 5. The defense mechanisms of living lungs are amazing and complex. Lung toxicity comes down to whether dusts can be cleared by these mechanisms without excessive inflammation and cell damage. photo: KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

NIOSH funded the studies because there was no better way, outside of longer-term exposure testing, to make a valid assessment of whether these emerging substitute materials were any safer than silica sand. It was implicitly understood that measurements of crystalline silica or the presence of any other toxic substances in the substitute materials could not be used on their own to provide a scientifically sufficient evaluation of their toxicity. It was presumed necessary to evaluate the substitute materials on the basis of exposure to the material dusts as a whole. The compositions of the individual abrasives were neither measured nor reported in these studies. The outcomes of interest were toxicological measurements in the lungs of the animals, which included biochemical markers for chronic inflammation and cell damage, along with pathological assessment of the animals’ lungs for evidence of fibrosis.

Strikingly, after comparing the results of exposure to the dusts of the alternative abrasives with the results of exposure to silica dust, seven out of the nine abrasive substitutes studied, were shown to induce chronic inflammation, cell damage, and lung fibrosis similar to or worse than silica sand, despite having widely varying chemical compositions and containing little to no crystalline silica. The chart in Figure 6 shows measured markers for lung cell damage relative to silica sand. The authors concluded that the persistent pulmonary inflammation and damage caused by the first seven abrasive blasting substitutes in Figure 6 suggest that they are not nontoxic alternatives to blasting sand. A follow-up exposure study of a subset of these alternative abrasives by the National Toxicology Program in 20203 confirmed the results of the earlier NIOSH studies.

Fig. 6. The bars show the markers for cell damage from each of the nine alternative abrasives included in the NIOSH studies1,2 as a ratio to the measured cell damage from silica sand.  Some degree of lung damage was observed within four weeks of a single exposure to the dusts of all nine alternative abrasives. chart: courtesy of the author

Exposure studies like these remain the gold standard for evaluating, quantifying, and confirming the toxicity of substances and mixtures. They provide the best possible data for evaluating whether an abrasive material dust is either toxic or could potentially cause cancer in workers from sustained exposure. The best way to know something is to measure it directly, which is what these exposure studies do.

Much More Work is Needed

While the results of the NIOSH and NTP exposure studies are certainly valid and informative, they were by no means extensive enough to develop dose-response relationships or to support conclusions about the toxicity or carcinogenicity of the alternative abrasives by agencies like ACGIH or IARC. The authors of these studies could only state that more exposure testing is needed.

It is for this reason that compliance with OSHA’s Toxic and Hazardous Substances Standards for monitoring and quantifying air concentrations of listed hazardous substances is still required. We have historically been and are still today left with the inferior method of monitoring workplaces for hazardous abrasive ingredients in the air. As we discussed in this article, and as the vast majority of toxicologists know, and as the NIOSH and NTP exposure studies clearly showed, the toxicity of the constituents of a complex mixture are not necessarily indicative of the toxicity of the mixture as a whole.

Workplace monitoring using air sampling and concentration measurements of listed, regulated substances can also be misleading for reasons beyond those discussed in this article. Published data and peer reviewed studies have uncovered fundamental flaws in the methods used for workplace air sampling that are specific to abrasive blasting environments. These flaws render air concentration measurements invalid and vastly inaccurate, yielding misinformation that has the unintended consequence of driving decisions that may not be in the best interests of workers.

The next article in this series will provide an overview of the published research, quantify the inaccuracies, and discuss the implications in both regulatory policymaking and workplace decision-making.

References

1. Hubbs, et al. Toxicological Sciences, Vol. 61, pp. 135-143 (2001).
2. Porter et al. Toxicology & Environmental Health, Part A, 65, pp. 1121-1140 (2002).
3. NTP Technical Report on the Toxicity Studies of Abrasive Blasting Agents Administered by Inhalation to Rats, NTP Tox 91, July 2020.