Since the introduction of the National Air Quality Index in 2014, New Delhi witnessed the highest number of days in the severe and near-severe category in November 2023. The Air Quality Index was 390 and above for 17 days – a figure on the higher end of the index that warns of potentially significant health complications with prolonged exposure. Beyond the national capital, India was the 8th most polluted country in the world in 2022 in terms of annual average PM2.5 concentration (μg/m³), according to the IQAir 2022 World Air Quality Ranking.

The catastrophic effects of air pollution on human health are widely discussed and often become a recurring topic of concern during the post-monsoon months when the country’s plummeting air quality raises alarm bells. According to a study by the Global Observatory on Pollution and Health at Boston College, the Indian Council of Medical Research and the Public Health Foundation of India, air pollution claimed 1.67 million lives in India in 2019 – the highest air-pollution-related death toll globally at the time. More recently, a modelling study by the BMJ revealed that outdoor air pollution causes about 2.18 million deaths annually in the country.

Beyond human health impacts, the scientific community has also flagged many hidden impacts of air pollution. In a research article published in July 2023, “Assessing the costs of ozone pollution in India for wheat producers, consumers, and government food welfare policies”, the authors show that India’s annual wheat yields declined by 14.18% between 2008 and 2012 due to air pollution from ground level ozone.

After carbon dioxide and methane, tropospheric or ground-level ozone is the third most important greenhouse gas. Long-term monitoring has revealed that India’s ground-level ozone has drastically increased between 2005 and 2020, and the upward trend will continue due to climate change and anthropogenic activities.

Along with health complications, the research article warns that the trend will also threaten cereal yields and aggravate associated economic losses.

The invisible threat to agriculture

The study adds to a growing body of research seeking to understand how air pollutants impact crops. In 2014, scientists from the University of California, San Diego, published an integrated study that looked at the impacts of both climate factors and air pollutants on crop yields.

In the research article, authors Jennifer Burney and V Ramanathan explain that short-lived climate pollutants such as black carbon and ozone directly impact the growth of crops by being toxic (ozone) or by altering the nature of solar radiation that reaches the plant (black carbon). Both these pollutants also warm the atmosphere, thereby contributing to climate effects on crop yields.

While black carbon is formed by incomplete combustion of fossil fuel and wood, ozone formation depends on existing air pollutants in the atmosphere. Biogenic volatile organic compounds or BVOCs (compounds emitted by plants under stressful conditions), carbon monoxide and nitrogen oxides react with sunlight to produce ozone. These pollutants are also released into the atmosphere due to biomass and fossil fuel combustion.

Burney and Ramanathan found that the combined influence of climate effects and air pollutants reduced India’s wheat yields by 36% in 2010, of which about 33% was contributed by air pollutants, highlighting the importance of understanding these influences in isolation. The authors explain that because these are short-lived pollutants (black carbon has an atmospheric lifetime of a few weeks and ozone of a few months), they are also ideal mitigation targets that can help improve agricultural productivity.

Similarly, in 2017, researchers from institutes across India estimated yield losses for wheat and rice based on ozone observations from 17 sites in the country and found that the highest losses for wheat yields occur in the northern parts of the country and for rice in the eastern parts of the country. Compared to Burney and Ramanathan, the 2017 research showed lower annual yield losses in the range of 4.2%-15% for wheat and 0.3%-6.3% for rice. The authors explain that the difference may be a result of the fact that the 2014 study is based on variations over a longer period.

Yield differences aside, all of the above studies underscore that climate effects do not reveal the entire picture when it comes to a decrease in crop yields – a fact that needs greater awareness, explains Environmental Science Professor Lisa Emberson, Department of Environment and Geography at the University of York, UK.

An environmental pollution biologist, Emberson has been researching the links between air pollution, climate change and agriculture for more than two decades. “Even within the scientific community, there is a lack of awareness across disciplines. The air pollution community is aware of the effects on vegetation but, for instance, the agronomy and forestry communities (ozone can also affect forests) are less likely to be familiar with pollution acting as stressors. This is slowly changing as scientists are beginning to work more across disciplines,” she explains.

Emberson adds that when it comes to the public and key stakeholders, awareness about the issue is still very low. “This is in part due to the fact that ozone is an ‘invisible’ pollutant – it’s not obvious as compared to, say, particulate matter. Its effects will vary yearly based on emissions and weather patterns and the effects may not be felt consistently over time,” she adds.

Source of air pollution

Lack of awareness is only one of the hurdles in establishing better mitigation measures to improve food security. The complicated nature of studying the influence of air pollution presents another problem.

In their research, Burney and Ramanathan explain that the lack of near-surface data for pollutants and mixing of black carbon with other aerosols makes it challenging to quantify the impacts of these pollutants.

Additionally, ozone levels tend to vary depending on the presence of other pollutants. While nitrogen oxides are precursors for ozone formation, when the concentrations are high, they also reduce the levels of ozone in the atmosphere. Further, ozone from one region could also move to another, thereby influencing the yields in locations away from the source.

The growing impact of climate change is adding to these concerns. Increasing temperatures accelerate ozone production, and with increased heat, there is little wind which, in turn, leads to the ozone concentrating in one region. Moreover, BVOC emissions also increase under these hot, stressful conditions, adding more precursors for air pollutants.

Emberson, one of the authors of the July 2023 study on the economic burdens of air-pollution-related yield losses, says that even before assessing impacts, merely finding the pollution levels across certain regions is a problem.

“Now we have good data for countries like North America, Europe, China, Japan, etc. Unfortunately, there is no reliable rural monitoring network across South Asia. This causes significant uncertainty in the assessment calculations and we have to rely on models to give ozone concentrations across these regions. Without data to evaluate those models, it gets complicated,” she adds.

Emberson’s research also shows that crop losses due to air pollution are higher under irrigated agricultural conditions as compared to rain-fed conditions, highlighting how we need to dig deeper in order to build tailored mitigation measures for protecting crops.

A thermal power plant in Karnataka. A recent study has highlighted that air pollution impacts crop yields and increases associated economic burdens. Credit: Pranav Kumar/Mongabay.

Finding better definitions

While measuring air pollution effects on crop yields is vital, researchers are also looking at how existing thresholds, that are meant to serve as a warning for harmful conditions, only take human health into consideration, neglecting other animals and plants.

Gufran Beig, a professor (Sir Ashutosh Mukherjee Chair) at the National Institute of Advanced Studies and Founder SAFAR (System of Air Quality and Weather Forecasting And Research), explains that air pollution norms often focus on the human health aspect of the problem, which makes it difficult to comprehend the enormity of the issue.

According to the National Ambient Air Quality Standards and Trends 2019, the permissible limit for ozone is 100 µg/m3 (about 50 ppb or parts per billion) over eight hours. But Beig explains that for plants, this limit is much lower. As early as 2009, Beig and other researchers from the Indian Institute of Tropical Meteorology, Pune, compared modelled and measured data for ozone levels and showed that values often exceed AOT40 (Accumulated exposure Over a Threshold of 40 ppb), which is harmful to vegetation.

“This reveals the need to have more stringent targets so you can also protect the crops,” Beig explains. He adds that yield differences vary between crops and locations where they are grown, which is why site-specific studies are important to gauge the extent of damage. In the above-mentioned studies, for example, researchers found yield variations between rice and wheat. Other studies have also shown yield variations for crops like maize and soybean.

Beyond impacts on yields, Beig says that pollution also physically damages crops, a fact that needs to be a part of all air-pollution related discussions. One study explained how pollution in the air leads to changes in plants, like yellowing and burns on leaves, shorter roots, and decreased leaf count, among other noticeable changes, highlighting the effects of other polluting components like sulphur dioxide and ethylene, to name a few.

While the direct impacts of air pollution are vast and varied, scientists have also observed links between respirable suspended particulate matter (RSPM – PM10, PM2.5 and PM1) and pollinators like the giant Asian honeybees. Over a period of three years, researchers looked at the effects of air pollution across various locations in Bengaluru on bee species and found that exposure to pollution affects the blood cells, heart rate, flower visitation behaviour and the survival of bees.

Similar to Beig’s observations on plants, Geetha Thimmegowda, lead author of the study, says that one of the primary goals of the research was to highlight that even permissible limits for humans are dangerous for other species. “We found that what was designated as interim air quality targets by the World Health Organisation for policy adoption were, in fact, harmful for bee populations. It might benefit humans to achieve the defined limits, but the bees would find it difficult to survive in those conditions,” says Thimmegowda.

While the research shows how air pollution indirectly affects food security by reducing pollination, Thimmegowda adds that it also raises concerns about the wider implications of the issue. As the founder of Nithya Haridhvarna — an organisation providing value-based research solutions for environmental issues, Thimmegowda says that awareness about the overarching issue is still very low, which makes it difficult to talk about the many layers through which air pollution affects us.

Addressing the issue

While mitigating air pollution is paramount, researchers also emphasise the need for immediate solutions to protect crops from polluting conditions. In the 2023 study, Pandey et al. suggest considering early sowing/maturing varieties so farmers can avoid peak ozone conditions. Limiting irrigation so the uptake of ozone is also limited is another mitigation measure to consider, the study suggests.

Thimmegowda says that as we rapidly expand our cities, we must also include green covers for more than one reason. “During our study, we also collected different plant samples from the study areas. When we looked at the leaves and flowers under a scanning electron microscope, we could see dust and other particulate matter blocking the leaf’s natural openings. This would affect photosynthesis and other plant pathways,” she explains. “We really should focus on green spaces and creating microclimates which will prevent dust and particles from moving around.”

But above all, it’s time to devise policies that also look at the larger picture when it comes to air pollution. “When you [a human] are in a high pollution environment, you instantly start coughing or reacting in some way. But with plants, you will not know about it until much later. We definitely need better policies to address the issue as these silent reactions are changing the ecosystems around us,” says Beig.

Adding to the discussion, Emberson says that implementing mitigation measures to reduce precursor emissions is crucial. “This probably means tackling nitrogen oxide emissions, which arise from burning fossil fuels for transport, power generation, and industries. In addition, taking action on methane emissions, an important greenhouse gas and ozone precursor, will also provide substantial benefits by slowing climate change,” she adds.

This article was first published on Mongabay.