By Kelly A. Reynolds, MSPH, PhD

Transcontinental travel is relatively simple—if you’re a microbe. While humans traveling internationally on airlines and other modes of transportation have contributed to the global spread of disease, microbes can easily hitch a ride around the world on a dust particle. But are these stowaways harmful to public health? If so, do they contaminate food and water supplies? Recent research sheds light on new discoveries of microbial transmission potentials and human health risks.

It’s a small world after all
According to NASA scientists and collaborators from the University of Maryland, 64 million tons of dust, pollutants and other particles travel across the Pacific Ocean to North America every year. To put this into perspective, a total of 69 million tons of these tiny airborne particles, or aerosols, are produced domestically by natural processes, transportation and industry. Thus, approximately half of the aerosols in North America originate overseas. Spring weather activity and increased wind speed aid in the transport of particles. While Asia contributes between 60 to 70 percent of the traveling dust plume, North America is also subject to migrant particles from Africa and the Middle East.

Hongbin Yu, lead atmospheric scientist collaborating with NASA on a study of intercontinental transport of aerosols, comments that countries share the atmosphere and that operations in one region can impact others far away. Despite dramatic increases in industrial operations in Asia, the majority of particles in global dust plumes are dust, not industrial pollutants. However, the impact of dust particles on climate, air quality and human health is complicated and not fully understood.

Tools of analysis
Through the development of NASA’s Earth-observing system, three-dimensional satellite pictures provide a distinct look at dust plumes and allow for quantitative measurement of the aerosol distribution and particle sizes and shapes. These detailed images now enable the distinction between dust and pollution components in traveling plumes. Further, new tools of genetic analysis allow for the collection and survey of DNA samples from globally transmitted dust plumes. Samples from two large dust plumes in North America that began in Asia in the spring of 2011 showed the presence of over 2,100 unique biological species. This rich species diversity exists in the troposphere, the layer of air closest to earth (18 km in middle latitudes) where the majority of weather occurs. The microbes identified were primarily bacteria and fungi that typically originate in soil. Some are known to be harmful agents to commercial crops but generally not harmful to humans.

Ecological impacts
During an interview with NASA’s Earth Science News Team, Yu stated: “Aerosols remain a wild card in climate change science because they can influence climate in several ways, and our current understanding is far from complete. In this study, we estimated that aerosols from overseas contribute about one third of solar radiation reduction, or solar dimming, in North America. Also, dust and pollution can change atmospheric circulation with a significant impact on weather and the climate.” In addition to stating that more research is needed, Yu said: “There are several other potential impacts that this study doesn’t address in detail. Dust and pollution aerosols can seed clouds and affect rainfall patterns. When dust and soot land on snow, the snow becomes darker and melts earlier. These influences can have important implications for water supply. Finally, changes in weather and climate will modulate the mixing and removal of pollutants and in turn affect air quality.”

Scientists have long recognized that microbes aid in the formation of ice crystals. This trait has been manipulated in agricultural applications where common bacteria are genetically engineered to prevent ice crystals from forming and protecting crops from frost damage. In nature, along with dust and other aerosol particles, microbes attract water molecules and enable the formation of ice crystals in the atmosphere at higher temperatures than occur with water alone. In 2008, researchers from Louisiana State University found microbes associated with at least a third of cloud ice crystals high in the atmosphere. Scientists at Montana State University recently dissected hundreds of hailstones falling to earth and found them teeming with bacteria. How increased concentrations of atmospheric particles affect this climatic condition is unknown but microbes in clouds are known to degrade pollutants and produce undesirable carbon dioxide compounds. Others have theorized about the possible role of African dust transported to the Caribbean and the deterioration of coral reef ecosystems. Although the transmission of dust may not be new, the addition of environmental contaminants, such as metals, arsenic and other industrial and human pollutants, have been on a steady increase.

“Dust and pollution aerosols can seed clouds and affect rainfall patterns. When dust and soot land on snow, the snow becomes darker and melts earlier. These influences can have important implications for water supply. Finally, changes in weather and climate will modulate the mixing and removal of pollutants and in turn affect air quality.”

Human health impacts
When asked if the imported dust and pollution aerosols impact air quality and human health, Yu stated: “Most of the particles that survive the trip across the Pacific Ocean are high in the atmosphere, above the air that humans breathe. Our study shows that only about five percent of trans-Pacific dust enters the continent within the lowest 1.2 miles (two kilometers) of the atmosphere. So air quality implications may not be significant, except in the western part of North America and during intense episodes. In such episodes, the large amount of aerosol import could elevate [particulate matter] concentrations to above the air quality standard, as recorded by previous observations. We believe that controlling near-surface domestic pollution is a more effective way of mitigating aerosols’ health effects.”

The most commonly isolated bacteria in recent dust plume samples were spore-forming species. Spore stages provide a mechanism for bacteria to persist in the environment and survive harsh conditions. How these well-adapted cells interact in the atmospheric ecosystem and settle in new regions is the topic of future research. Perhaps the most compelling evidence of human health impacts of global dust plumes is that areas most highly impacted also have the highest rates of asthma. Numerous studies have found an association with seasonal dust storms and increased allergies, asthma and silicosis/pulmonary fibrosis risk. US researchers further isolated 95 bacteria from dust events in Bamako, Mali. Of these, five percent were potential plant pathogens and 25 percent were opportunistic human pathogens, including Acinetobacter, Staphylococcus and others.

Final thoughts
Research confirms that dust plumes travel the globe in a matter of days and are rich in microbial diversity, including unique microbial species and potentially harmful pathogens. How these microbes interact in the troposphere and how they settle into environments of human contact (i.e., air, soil, food crops and water) is unknown. Human health risks have only been minimally identified. Thus, the next decade promises to bring new insights into the microbial world that travels miles above us.


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About the author
Dr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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