Over the past 100 years, Earth’s average temperature has increased by more than 1.5ºC, and the 10 hottest years ever recorded have all occurred since 2005. This seemingly small increase in temperature has caused major environmental changes, including melting glaciers, sea level rise, and intense weather events.

In addition to the more obvious effects on the environment, climate change is also affecting human health, including increasing the risk of infectious diseases. At a recent presentation for Chan Zuckerberg Biohub scientists and staff, Biohub Investigator Chaz Langelier, associate professor of medicine in the Division of Infectious Diseases at UC San Francisco and a long-time collaborator with the Biohub’s Infectious Disease initiative, discussed these troubling trends.

Warmer global temps have increased the prevalence and spread of waterborne diseases, zoonotic diseases, and respiratory infections—especially those tied to air pollution from wildfires, for example. But not all hope is lost, Langelier said. Renewable energy sources are steadily becoming more affordable and more widely implemented, which will help to dampen the overall impact of human-caused climate change.

What’s in the water?

Shellfish lovers beware: Warming sea temperatures may be increasing the likelihood of poisonous oysters, clams, and mussels hitting your plate. The culprit is Vibrio vulnificus, a species of bacteria that thrives in warm oceans. V. vulnificus can enter the body through an open wound if swimming in infested water, or through the consumption of raw or undercooked seafood. Infection is severe and potentially fatal.

“The incidence of Vibrio has a direct relationship to climate,” Langelier explained. Studies have found that “as water temperature goes up, you’re more likely to have this bacterial pathogen show up in oysters,” because these harmful bacteria thrive in areas with lots of algae growth, and algae thrive in warm, nutrient-rich water.

Over the past 40 years, the coastal area suitable for Vibrio infections has increased by 50% in northern latitudes, according to a recent review in The Lancet, Langelier said. “And only recently have Vibrio infections been described in Alaska.”

Although V. vulnificus infections are still relatively rare, increases in the infection rate of another Vibrio species—Vibrio cholerae, which causes cholera—is of much greater concern in many parts of the world, Langelier said, because of lack of access to clean drinking water. “In a lot of regions of the world, people just don’t have the ability to accommodate an increased burden of infectious waterborne disease that could result from climate change.”

That includes certain areas of the U.S. For example, in San Francisco in 2015, contaminated water carrying Shigella bacteria —which causes dysentery—led to a particularly problematic and deadly outbreak among the city’s homeless population. Researchers found the outbreak was tightly correlated with extreme precipitation events associated with climate change, which in turn led to an increased exposure to contaminated water by unhoused people at that time.

Break out the bug spray

Another way climate change is impacting the spread of disease is through mosquito-borne illnesses, as more and more areas of the globe become suitable habitat for these disease-carrying pests.

One of the most recent examples is the emergence of West Nile virus in the U.S.—a disease that arrived on our shores relatively recently. In 1999, experts identified the first outbreak in New York, affecting only four patients. But the virus steadily spread across the rest of the country, and by 2014, West Nile had become prevalent throughout the contiguous 48 states.

“Researchers have speculated about what might have led to this emergence and westward distribution,” Langelier said, “and there are data to suggest that climate-related factors played a role.”

Modeling studies examining the odds of West Nile spreading across the U.S. have found that warmer temperatures with heavy rain events that allowed for standing water were optimal for mosquito replication and the spread of the West Nile virus. Scientists have documented similar trends with mosquitoes that carry malaria and dengue.

Tick-borne diseases, such as Lyme disease and rickettsial infections, are also on the rise and becoming more prevalent in higher latitudes. The Centers for Disease Control and Prevention reported a two-fold increase in the incidence of tick related-infectious diseases between 2001 and 2014, and similar trends have been documented in Northern Europe and Canada, Langelier said.

Trouble in the air

Around the globe, air pollution is estimated to cause 7 million premature deaths annually. “It’s a huge and largely preventable public health problem,” Langelier said, adding that there are two main pollutants of concern when it comes to human health and climate change, he said: ozone and particulate matter.

Ozone offers us protection from radiation in the stratosphere’s ozone layer, but down at our level it’s a big problem for human health. Ozone is produced by combustion reactions, including those that happen in a car’s engine, or from the reactions that occur when heat and sunlight come in contact with nitrogen oxides, such as those found in paint and other volatile chemicals. It’s a so-called free radical, meaning that it’s very unstable, and if inhaled, it can cause respiratory illnesses by reacting with the tissue that lines the inside of the lungs. Ozone production increases with temperature, and a hotter climate will mean a greater number of days with unhealthy levels of ozone in many U.S. cities.

Particulate matter (PM) is the term for fine particles, smaller in diameter than the width of a single human hair, that are byproducts of combustion. PM is generated in significant amounts by forest fires, which become more frequent with warmer, drier climates, as well as by fossil-fuel combustion, which also contributes to climate change from CO₂ production.

“These particles often have metals and other organic compounds that are adsorbed to them, which when inhaled, can lead to a number of different inflammatory reactions in the lungs,” Langelier said.

Several studies have reported links between air pollution and disease prevalence. For example, a recent study examining the incidence of lower respiratory infections in more than 145,000 children in Utah found that the odds ratio for getting an infection increased within one week of a spike in particulate air pollution and peaked three weeks after that spike. During the 2020 and 2021 fire seasons, many cities in California and the Western U.S. experienced daily PM levels 10 to 50 times greater than the safe threshold established by the U.S. Environmental Protection Agency.

“There are also quite a few studies now that provide fairly convincing evidence of a link between PM air pollution and COVID,” Langelier said. For example, a 2020 study found that a very small increase (only 10 micrograms per cubic meter) of extra-tiny particulate matter (>2.5 micrometers in diameter) in the air was associated with an 11% increase in COVID cases over the four weeks following that meager increase.

But a burning hope, too

Despite all of the dire evidence linking human illnesses to climate change, there are reasons to be hopeful, Langelier said.

“One of the most exciting things right now is the profound decrease in costs of renewable energy,” he said. “Amongst the top things that can be done to mitigate climate change is shifting from combustion-related energy production to renewable energy.”

Legislation has also made a difference, he said. “The Clean Air Act, which was implemented in the 1970s, has actually been found to have significantly increased life expectancy.” Indeed, a 2009 study found that the reduction in particulate matter air pollution between 1980 and 2000, as a result of the Clean Air Act, significantly increased life expectancy.

“So in terms of being able to alleviate the burden of climate change and its related health effects,” Langelier said, “we should be concerned, but feel hopeful at the same time.”