All of the world's cities with more than 1 million people are consuming much more oxygen than they produce — hundreds or thousands times more in some cases — and may soon be at risk of exposing their inhabitants to harmful low-oxygen levels in very calm weather, according to new scientific modeling.

Published April 27 in Environmental Science & Technology, a study compiled some of the first estimates of oxygen flow within cities. Earlier research has shown that oxygen deficits are driven primarily by fossil-fuel use.

More unsustainable oxygen consumption is associated with hotter temperatures and more excessive water usage, the paper's authors found, while some urban areas could reach unhealthy oxygen levels in a matter of days without any wind. The researchers recommended that cities increase vegetation and lower population density to alleviate their oxygen imbalance.

Despite oxygen's critical function for human life, how human activity affects oxygen levels is understudied, according to Jianping Huang, the lead and senior author of the new paper and a professor of atmospheric sciences at Lanzhou University.

"Compared with the increase in carbon dioxide, the current decline in oxygen has received little attention," Huang and Yun Wei, a Ph.D. student studying atmospheric sciences at Lanzhou University and another author of the paper, said in an email.

Constituting 20.9% of the atmosphere, oxygen gas cycles through the environment via many kinds of interactions, but living things are the dominant drivers of its production and consumption. Oxygen is primarily produced as a waste product during photosynthesis, when plants, algae, plankton and other life absorb light and carbon dioxide. Most forms of animal and bacterial life consume oxygen and release carbon dioxide during respiration. 

But the burning of fossil fuels is the leading oxygen consumer on Earth, and, consequently, the gas is being depleted faster than it is being replenished. Combustion used up 73% of all oxygen consumed between 1990 and 2005, Huang found in a 2018 analysis — 56% more than what plants produce, and eight times more than what humans breathe. 

And under a worst-case scenario for climate change, atmospheric oxygen levels will fall by 0.12% to 20.83% by the end of the century, according to that analysis. The authors said the lowering concentrations would increase the risk of below-normal levels of oxygen, known as hypoxia, on land and in the oceans.

In the new paper, Huang and his co-authors focused on oxygen levels in cities, where concentrations can fall more rapidly, given dense populations, high fossil-fuel use and low vegetation cover compared with nonurban regions. Trees save lives because they lower temperatures by providing shade and transpiring water, but the benefit is spread unequally: Recent research found that low-income neighborhoods in U.S. cities have less tree cover than high-income areas.

"As the area with the most serious oxygen imbalance, cities must be the starting point and focus of attention to hypoxia," Huang and Wei said.

The modeling focused on oxygen flows within the 391 cities with a population of 1 million or more, which represent nearly half of the world's urban population. The researchers used demographic data of the cities to model how their oxygen was produced and consumed.

The researchers assigned cities an oxygen index, a ratio between a city's consumed and produced oxygen. If a city's oxygen index was greater than 1, it was creating an oxygen deficit.

They found that between 2001 and 2015, the oxygen index of the cities ranged from about 4 to 3,454, and the average was 59 — meaning the world's largest cities collectively used up 59 times more oxygen than they made. Shenzhen, China, the world's most populous city, had an index of 457.9, while the Egyptian capital, Cairo, clocked in at 2,430.6.

Oxygen indices were higher in larger cities: 75% of those with more than 5 million people had an index above 100 compared with 53% of the overall sample. Other factors affected the oxygen consumption of urban areas, such as vegetation coverage: New York, with an index of 161, has a higher population but also more greenery than Los Angeles, whose index was 240.4.

Globally, the oxygen index of the 391 cities decreased between 2001 and 2010 before rapidly growing through 2015, the latter trend driven by more fossil-fuel consumption everywhere except Europe and North America, which are highly industrialized regions.

Cities' oxygen deficits are at least partly alleviated by air flowing from rural areas or wilderness, but oxygen levels could stay low in the face of extremely calm weather, according to the researchers. Under these conditions, they calculated that a city with an oxygen index of 1,722 could reach the hypoxic conditions of 19.5% oxygen concentration in 10 days; with an index of 3,000, it would get there in a little more than half the time.

"Global warming has led to frequent occurrence of extreme calm weather in cities, and the accumulation of pollutants has led to frequent occurrence of urban haze," Huang said. "In this case, without outside oxygen delivery, oxygen levels in lower layers of cities would rapidly drop to levels that may threaten human health."

Cities with a high oxygen index also consume water more excessively, increasing their risk of water shortages. And they suffer more from other environmental pressures. These cities are more likely to be exposed to higher temperatures, as well as more heat waves that last longer. Recent research found that climate change and population growth along the East and West coasts will drive a doubling of heat stress in the U.S. by 2099.

Huang and Wei said that to reduce oxygen consumption, cities need to lower their population density and transition to forms of energy production that do not consume oxygen, such as renewable energy sources. Adding more vegetation is the primary way for urban areas to boost oxygen production, they said.

Despite these health threats, monitoring of oxygen levels in urban areas is rarely performed, according to Huang, and publicly available long-term data does not exist. He and his lab plan to begin collecting data on oxygen concentration in cities across the world to confirm the threat of hypoxia and create a scientific starting point for a transition to sustainable oxygen levels.

"Not only in cities, but in our entire Earth system, the oxygen cycle is gradually losing balance under the interference of human activities," Huang said. "We hope our research can arouse people's attention to the modern Earth's oxygen cycle."

The study, "Declining oxygen level as an emerging concern to global cities," published April 27 in Environmental Science & Technology, was authored by Yun Wei, Jianping Huang, Xiaoyue Liu, Dongliang Han and Linli An,  Lanzhou University; Jianguo Wu, Arizona State University; Haipeng Yu, Chinese Academy of Sciences; and Jiping Huang, Enlightening Bioscience Research Center.