Zoe Berg, Senior Photographer

In a recent study, an international team led by Yale researchers found that deaths linked to ground-level ozone will likely increase unless the world meets the Paris Agreement goal. 

Conducted by scientists from the School of Public Health, the Climate, Health and Environment Nexus, or CHEN, Lab and other international institutions, the research spans 406 cities across 20 countries from 1985 to 2015. Using data from the Multi-Country Multi-City Collaborative Research Network, a global collaboration of environmental and health scientists, the researchers analyzed the acute impacts of short-term, daily exposure to ground-level ozone. 

After establishing this baseline, the researchers modeled future ground-level ozone mortality. They found that deaths will increase from 2010 to 2050 except in the climate scenario consistent with the Paris Agreement, which calls for the global temperature to rise no more than two degrees Celsius by the end of the century. 

“If we are taking actions to address climate change, we are saving lives,” said Kai Chen, an assistant professor of epidemiology at the School of Public Health.

Ground-level, or tropospheric ozone, forms when nitrogen oxides and volatile organic compounds react in sunlight and high temperatures. As global warming increases the number of hot days per year, the production of this hazardous ozone is projected to rise.

The model explored various emissions scenarios outlined in the Intergovernmental Panel on Climate Change Sixth Assessment Report (IPCC AR6). The ozone-related mortality fraction — the excess number of ozone-related deaths divided by the number of total deaths — only decreased in the low-emissions scenario aligned with the Paris Agreement target. Still, the fraction only minimally decreased from 0.17 percent to 0.15 percent.

According to Chen, the researchers used the best available global climate models and accessed daily projections downscaled to the city level to obtain more granular results. For Alexandra Schneider, the deputy director of the Institute of Epidemiology at Helmholtz Zentrum München, it is important to also consider the “heat island” effect in urban areas, where cities generally have higher average temperatures, meaning more tropospheric ozone is produced and there are higher ground-level ozone-related mortality rates. She plans to continue researching the impact of city size on increased temperatures and ozone-related mortality rates in the future. 

The researchers also found that the ozone-related mortality rates continued to rise in big cities in Mexico, China and the United States — even in the climate scenario that meets the Paris Agreement goals.

In future studies, Chen hopes to consider other pollutants and incorporate long-term exposure data. He also seeks to get data from additional locations — including many areas across the developing world that do not have extensive ground air quality monitoring stations networks — by using satellite monitoring models. 

Both Schneider and Chen also plan to assess in more detail how mortality changes over time, accounting for factors like population growth, emissions increase or reduction, life expectancy changes and shifts in medical treatment and food intake. 

“This model is just using mortality, [but] this is just the tip of the iceberg,” Schneider said.

Schneider emphasized the need to understand better the underlying mechanisms behind the interaction between rising temperatures, increasing emissions, changing baseline mortality rates and population size shifts on growing mortality. 

“Now you only see that mortality increases but don’t know how you can treat it except making the environment better,” Schneider said. 

Cate York ENV ’24 is a second-year master’s student at the School of the Environment. From Los Angeles, a metropolis with pollution and poor air quality, York emphasized the importance of spreading awareness about ozone and mortality.

“As an individual, I don’t know what to make of it or what to do about it. I don’t even know what levels I was exposed to in the area where I lived,” York said. “When you are in it, I don’t know if people even realize the extent of the problem. That’s just their baseline.” 

According to the State of Global Air, ground-level ozone contributed to 365,000 deaths worldwide in 2019.