Particle size is key factor in coal mining-related respiratory disease

One of the major occupational health hazards for coal workers in the U.S. is coal mine dust-related respiratory diseases. New findings by Penn State researchers shed light on the causes of respiratory diseases related to coal mine dust.

"This project is the first of its kind to look at what is really contributing to the recent increasing trend of pneumoconiosis and silicosis in some regions in the United States, and the increasing trend of progressive lung fibrosis over the last decade," said Shimin Lui, associate professor of energy and mineral engineering in the John and Willie Leone Family Department of Energy and Mineral Engineering at Penn State.

Previous studies have shown that inhalation of coal mine dust is known to cause several types of respiratory diseases, the most common being pneumoconiosis (CWP). The Mine Safety and Health Administration (MSHA) and other government entities have devoted substantial efforts to reduce the adverse effects of dust on miners' lung disease, but even with strict MSHA regulations, since 2000, the rates and severity of CWP have consistently increased.

Reducing the permissible amount of exposure to respirable coal dust concentrations alone may not be effective because evidence suggests that excessive dust concentrations may not be the only root of CWP, Liu explained. Instead, its toxicity, composition and size could be significant factors for the miners' lung diseases, he added.

Prior research has shown that nano-sized particles are more toxic than their larger counterparts due to their unique physicochemical properties and easier uptake by living organisms, but the underlying mechanism of miners' lung diseases due to nano-coal-dusts (NCDs) exposure is not yet well understood nor are the key factors resulting in CWP known, Liu said.

To close these knowledge gaps, Liu and his team worked closely with partner mines to collect fresh coal dust at different working locations. They then characterized physical, compositional and petrophysical properties of the coal dusts using techniques including scanning electron microscopy/energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Finally, the samples were sent to Carnegie Mellon where the coal dust particles were exposed to a novel lung cell device to monitor the biological effects. The results were recently published in the Journal of Hazardous Materials.

"We wanted to know what the indicators are that might be linked to the toxicity," Liu said.

Through their work, the team successfully established a reliable, reproducible protocol for fully characterizing the potential of toxicity in the coal dust, which included parameters such as mineralogy, pore size distribution and the geochemical perspective.

The results showed that several factors, including size, were important. Compared to larger coal dust particles (micron-scale), the nano-coal dust showed an increase in carbon content and aromaticity and a decrease in oxygen content along with the reduction of oxygen-containing functional groups. Pore volume and surface area had more than a five-time increase for the nano-coal dust. The reduction of oxygen content suggests a decrease in the ability of a water droplet to maintain contact with a solid surface, otherwise known as it's wettability. The increased pore volume and highly enhanced surface area in the coal nanoparticles along with weaker wettability suggest that nano-coal dusts have the potential to have significantly more toxicity when inhaled by coal miners.

"We found that nanoparticles have the same toxicity as silica," Liu said. "Many think progressive fibrosis is from silica, but really the coal nanoparticles may also contribute to the toxicity. This is the first study of its kind to understand nano-particulate toxicity."

Liu hopes that this discovery will influence U.S. regulation and advance industry implementation of personal protective equipment that can better filter coal dust nanoparticles.

Moving forward, Liu says they will continue to explore NCDs and hopefully develop a national data survey for the nanoparticle contribution, which is still significantly unknown. Second, they want to lead research and instrumentation innovation.

"We know the different sizes matter, and the best practices from the mining industry are still valid, but we want to develop some wearable tools that can give the nanoparticle range of the dust exposure that is contributing to the long-term impact for the workers' health because, really, the miners are the ones who bear that health burden," Liu said.

More information:
Rui Zhang et al, Characterization of nano-to-micron sized respirable coal dust: Particle surface alteration and the health impact, Journal of Hazardous Materials (2021). DOI: 10.1016/j.jhazmat.2021.125447

Provided by Pennsylvania State University