Q&A: Carl Zimmer on Airborne Diseases: 'These Are All Big Concerns'

Our air quality plays an important role in maintaining the health of both our planet and the general public. In his new book, Air-Borne: The Hidden History of the Life We Breathe, award-winning science journalist and author Carl Zimmer dives deep into the science behind aerobiology—identifying how microorganisms, pollutants and viruses occupy the air around us. In this Q&A, Zimmer lends his expertise to shed light on the dangers of airborne pathogens, pollutants from wildfire smoke and the potential of biological weapons.

What most surprised you while researching this book? During the pandemic, scientists sparred over how COVID spread. Some researchers said it was airborne, but public health authorities like the World Health Organization flatly said it wasn't. It took two years for the WHO to publicly turn around. This seemed very strange to me at the time as a journalist. I was very surprised to discover just how much of our basic understanding of airborne transmission had been worked out almost a century ago. Unfortunately, those lessons got forgotten, along with the pioneers who learned them. Should we be concerned about any specific airborne viruses now? Some familiar diseases out there are mainly or solely airborne. Tuberculosis is one. Measles is highly airborne as well. For other common diseases, the picture is not yet clear—mainly because there hasn't been enough focus on research that can clearly work out how pathogens are getting from person to person. Influenza may be able to spread by contact with contaminated surfaces, with short-range coughs flinging droplets at people's faces and also by long-term airborne transmission. But scientists are still running experiments to figure out how important each channel is. That's something we really need to understand better—especially with H5N1 threatening to jump from animals to people these days. Some diseases can travel through the air over staggering distances. Valley Fever, for example, is caused by a fungus that gets kicked up from the soil by erosion and wind and can be blown for hundreds of miles. And climate change is predicted to loft more of these fungi into the air in the years to come and cause more cases of Valley Fever in the United States. These are all big concerns. But perhaps the biggest of all is what scientists call Disease X: a pandemic pathogen that now circulates among animals but will jump over sometime in the future. If it becomes airborne, it will be all the more difficult to fight. We know this already, because of the surprises that COVID delivered back in 2020. How much should we worry about air pollution on a day-to-day basis? Air pollution from sources like cars and coal-fired power plants takes a mind-boggling toll on the world's health. In a recent study, scientists estimated it kills 8 million people annually. Air pollution deaths in the United States have dropped a lot as the country has cleaned up its air. But it's still responsible for 48,000 premature deaths.

What effect do wildfires like the recent ones in Los Angeles have? Wildfires pump vast numbers of microbes high into the atmosphere. That may be hard to believe—shouldn't wildfires just kill everything in their path? It turns out that life can survive in the intense updrafts created by fires. They can pull bacteria and fungi off of leaves and bark, and even out of the ground. They become airborne and can soar miles into the sky. Then the winds can blow them for vast distances before they settle back to Earth—or get inhaled by us. What about long-term health implications of smoke and fires? Wildfire smoke can deliver a huge dose of air pollution in very little time. It has been linked to a range of conditions, from asthma to heart disease. And when wildfires hit cities like Los Angeles, they don't just burn plants. They burn plastic, paint and all sorts of other artificial materials—which create their own set of harms when people breathe them in.

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Should airborne biological weapons be a concern? How can we protect ourselves from bad actors? In World War II, the United States military borrowed pioneering research on aerobiology to design new kinds of weapons. They investigated anthrax bombs that could be dropped from planes and giant hoses that naval ships could use to spray germs on coastal cities. They continued to build biological weapons for decades after the war. But Americans were not the only ones: The Soviet Union worked even longer on biological weapons, experimenting with smallpox and other highly lethal pathogens. These germ warfare programs raised fears that a rogue country or terrorist cell could start an epidemic that would kill millions of people. But that has not happened. Historians have concluded that the greatest toll from biological weapons over the past century took place in China, where the Japanese army unleashed germs on soldiers and civilians. But their attacks probably only killed tens of thousands of people at most. The fear of biological weapons—fueled by breathless books and movies—may have caused more harm. The Bush administration justified starting the Iraq War in part with claims that Saddam Hussein had a massive stockpile of biological weapons. He didn't. But Bush invaded Iraq anyway, and the war led to hundreds of thousands of deaths anyway. What's a favorite fact about air? It's crazy how much we don't yet understand about the life in the air. Scientists have long suspected that insects migrate in big numbers through the sky, for example. But until recently they couldn't watch the bugs in flight. Last year, a team of scientists used radar to scan for insects above a few hundred square miles of China. They found over 9 trillion insects soaring through the sky each year. That's 15,000 tons of insect biomass—just in one corner of China!