A hitherto underestimated and, therefore, understudied, aspect of global warming is its indirect impact on tuberculosis incidence and the potential of a double whammy to some of the poorest sections of society, say scientists.

With increasing pointers from tuberculosis experts towards the indirect links between climate change and tuberculosis (TB), the World Health Organization (WHO) in October 2024 held a two-day consultation to draw up a research framework on climate change and TB. According to Matteo Zignol, unit head at WHO’s global tuberculosis programme, climate change and health has emerged as “one of the hottest topics in global health”.

“TB is off the radar in climate change debates,” he stated at an annual Union World Conference on Lung Health in Bali in end-2024.

WHO is now working on a framework that will address three large research domains: migration and displacement; food and water insecurity; and health systems disruptions that are being increasingly seen during recent flooding events, and was evident during the COVID-19 pandemic.

Zignol told the 2024 Bali conference that the link between climate change and TB becomes more clear when one considers TB determinants, especially under-nutrition. He cited a 2024 Cochrane Review published in 2024, which shows that undernutrition increases the risk of TB two-fold in the short term or less than 10 years; and may also increases the risk of TB in the long term, or more than 10 years. Zignol said that “there have been projections showing that a two-degrees Celsius increase in the world temperature could translate into 189 million people who go to bed hungry. In a four-degrees Celsius warmer world, this could reach a staggering 1.8 billion people.” Currently, there are no ongoing to planned studies on global warming and TB incidence, in India.

However, the impacts of climate change on food security may be more important in the medium to long term, says Soumya Swaminathan, chairperson of the M S Swaminathan Research Foundation and former chief scientist at the WHO. “Already, the number of food-insecure people has increased, especially in Africa, and this is likely to increase further,” she says. If under nutrition increases, definitely TB will go up because it is the top risk factor for TB globally and accounts for 40% to 50% of cases, she says.

The other link is air pollution and TB, adds Swaminathan. High PM_2.5 (particulate matter that is fine, inhalable and less than 2.5 micrometres in diameter, for example, soot) in many developing countries is also a risk factor for TB. “Actions to reduce air pollution will also help mitigate climate change as sources are the same,” she says.

Modelling studies using different emission levels of greenhouse gases and shared socio-economic pathways (SSP) offer some valuable insights, according to Rebecca Clark, research fellow at the department of infectious diseases and epidemiology at the London School of Hygiene and Tropical Medicine (LHSTM). These scenarios used in modelling studies in climate research explore how global society, demographics, and economics might evolve over the 21st century, affecting greenhouse gas emissions and climate change determinants. Clark’s team’s modelling studies show that climate change can impact a major determinant, undernutrition, which in turn “will likely have a significant impact on the TB burden.”

Extreme weather and climate change are driving food prices up, says Clark. “Modelling can estimate the impact of undernutrition due to climate change on TB burden,” she said, while speaking at the Union World Conference on Lung Health in 2024.

Uzma Khan, medical director at the Singapore-based Interactive Research and Development (IRD) Global, tells Mongabay India that people at risk due to climate change and TB are among the most vulnerable sections of society. Both problems are linked to social inequity issues, both in the Global South, for example the 2023 devastating floods in Pakistan, and in the Global North, for example, higher incidence of TB in indigenous communities in Canada. “The problem is not restricted to the global South or LMICs (low and middle income countries).”

Khan says that there is a need to break down silos to address climate and TB injustices. One should not view them just through biomedical or academic lens which limits “our understanding and hinders solutions,” she says. “It requires a shift in mindset,” she adds.

“Addressing these complicated issues requires new ideas and investments collaborative action across sectors,” says Khan. “TB activism could learn from climate activism.”

These recent warnings add to recent research reports on the links between climate and TB. In 2022, researchers from the University of Adelaide reported that climate change is “likely to affect the susceptibility of individuals to TB by increasing the prevalence of its underlying risk factors, which include undernutrition, poverty, overcrowding, indoor air pollution, and HIV infection, diabetes. Their report in Environmental Research says that “developing countries are vulnerable to the impacts of climate change on health” and that “tuberculosis should be recognised as a climate-sensitive disease”.

“This is an evolving field of research that requires further attention in the scientific community,” the researchers noted.

“Based on our findings, it is likely that climate change will exacerbate food insecurity and subsequently increase the risk of TB transmission in vulnerable communities,” Sahil Kharwadkar, one of the authors of the Environmental Research paper, who is currently junior medical officer at the Wagga Wagga Base Hospital in Australia, tells Mongabay India.

“In our review, we identified that extremes of weather, notably floods and droughts, were strongly associated with undernutrition – an important risk factor for TB,” says Kharwadkar, The greatest evidence for this association was in children, who are even more susceptible to TB infection, he says.

The likely mechanism is through the impact of temperature and precipitation extremes on crop production and subsequent food availability. “Our sample included four studies with data from India and the remaining 13 studies examining undernutrition were based in developing countries across Asia and Africa,” adds Kharwadkar.

Despite the extensive progress made towards TB eradication, there remains a significant number of deaths attributed to TB in vulnerable populations, points out Kharwadkar. India continues to suffer from a high TB burden and is estimated to account for a quarter of global TB cases. “Moving forward, it is essential for national health policy in India to acknowledge the role of climate change in TB prevalence and focus on mitigating its effect on food security.”

Another review from 2021, by researchers at the Japan-Nepal Health and Tuberculosis Research Association reported in the International Journal of Biometeorology that climate change affects tuberculosis through diverse pathways: changes in climatic factors like temperature, humidity, and precipitation influence host response through alterations in vitamin D distribution, ultraviolet radiation, malnutrition, and other risk factors.

The rise in extreme climatic events induces population displacement resulting in a greater number of vulnerable and risk populations of tuberculosis, the Japan-Nepal report says. It creates a conducive environment of tuberculosis transmission and development of active tuberculosis and disrupts tuberculosis diagnosis and treatment services.

The report adds that, further studies and novel methodologies are required to address such a complex relationship and better understand the occurrence of tuberculosis attributable to climate change.

A separate report, also in 2021, based on a scoping review by researchers in Indonesia, reported that climate change creates new transmission opportunities for airborne infections in warm temperature.

Meanwhile, Stephan Schwander and colleagues at the department of urban-global public health at the Rutgers School of Public Health, Newark in the U.S., are conducting a meta analysis and systematic review on air pollution and tuberculosis. According to their preliminary analysis, which was presented at the 2024 Union World Conference on Lung Health, children are at higher risk to the impacts of indoor air pollution and an increase of 10 microgram per cubic metre of ambient fine polluting particles, such as particulate matter (PM_2.5) or soot, increases TB risk by nine percent.

Schwander says that climate change is affecting air quality, resulting in changes in air pollution meteorology, such as ventilation, dilution, precipitation and other removal processes for circulating polluting particles. Higher ambient temperatures also increase ozone formation and fine particulate matter PM_2.5, as well as the frequency of sand and dust storms in arid regions; and wildfires.

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Banner image: A doctor in a hospital in West Bengal assesses the X-ray of a tuberculosis patient. Image by ILO Asia-Pacific via Flickr (CC BY-NC-ND 2.0).