Between the 1860s and 1920, successive outbreaks of typhoid fever killed over 300,000 Americans. As population growth surged and people moved to urban areas en masse, American cities began to dump sewage in the same rivers that provided their drinking water. After epidemiologists linked typhoid outbreaks to water cleanliness, cities began building large-scale sand filtration systems in the 1890s, and in 1908, Jersey City pioneered the first continuous chlorination of a public water supply. By the 1920s, typhoid deaths had fallen by two-thirds, and waterborne diseases were in retreat across the country.
While typhoid and other waterborne diseases triggered vast engineering and regulatory responses, the equivalent airborne threats have not. Tuberculosis alone kills more than a million people every year around the world, yet the air in schools, clinics, and public buildings remains largely unfiltered and unmonitored. Covid-19, which killed over seven million people, demonstrated how rapidly airborne pathogens can spread in poorly ventilated spaces.
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Just as filtration and chlorination made drinking water safe at scale, we now have the tools to do the same for indoor air: ventilation, high-quality filters, and germicidal light. A century ago, germicidal light at 254 nanometers seemed to be a promising way of controlling pathogens by killing them in the air, but it turned out to cause irritation and cancer in the skin, and it was largely dropped when antibiotics became widespread.
But today there is an update that has none of these drawbacks. We now know that wavelengths under 230 nanometers, especially 222 nanometer light, are harmless to humans, but can still disable microscopic pathogens. We know how to filter out all wavelengths except the ones we want, and how to direct them away from humans, cycling the air through them to clean it without exposing people to it, just in case it carries unknown risks. This far-UVC light, as it is called, may be how we can make the air we breathe as safe as the water we drink.
The sterilamp
Until the mid-nineteenth century, most physicians believed that disease spread through miasmas, poisonous vapors rising from filth and decay. Contemporaries were obsessed with air: moving to the countryside for the air, taking the air at the seaside. Bad air was blamed for sickness. Herbs were burned to purify the air to fight the plague. But because they didn’t understand what made the air dirty, they were not very good at cleaning it. This began to change only when Louis Pasteur and Robert Koch provided the first definitive proof that microscopic organisms were responsible for infectious disease.
Cleaning air relies on the same fundamental techniques as cleaning water: replacement, filtration, and disinfection. Pathogens replicate inside people, who then expel them into the air by breathing, talking, or coughing, where they can remain suspended and infect new individuals. The relative contributions of aerosol transmission, droplet transmission, and fomite (surface) transmission vary between diseases. Certain diseases, such as Covid-19, are driven by a small number of highly infectious individuals (‘superspreaders’) that account for a disproportionate number of cases.
In 1877, British researchers Arthur Downes and Thomas Blunt stumbled upon a discovery that would lay the groundwork for modern disinfection. In a paper submitted to the Royal Society of London, they described how over the course of six months they had used sunlight to prevent bacteria from growing in a tube.
Follow-up research by Robert Koch demonstrated that sunlight could kill Mycobacterium tuberculosis, but the early experiments lacked precision. Scientists knew light worked, but not which parts of the spectrum were responsible. The turning point came in 1930, when Frederick L Gates published the first quantitative analysis of how ultraviolet light affected bacteria, pinpointing peak germicidal effectiveness at 265 nanometers, the same point that nucleic acids – DNA and RNA – absorb light.
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