In November 2023, Delhi witnessed a dramatic deterioration in air quality, casting a haze that obscured the city's landmarks and posed serious health risks to its millions of residents. This alarming phenomenon was not the result of industrial accidents or uncontrolled urban fires but a recurring preventable cause: the extensive burning of crop stubble by farmers in the surrounding regions (1).

Delhi is not a unique case. Every year, farmers all over the world burn the leftover stubble from crops for many reasons. While vegetation burning can offer some agricultural benefits, smoke from both regulated and unregulated burns also results in toxic clouds of smoke which lower visibility, causes coughing and breathing problems, and pollute the landscape for miles beyond their source.

What is crop burning?

Intentional crop burning is a widespread agricultural practice, employed globally by farmers to manage fields after harvests. By burning leftover stubble from crops like summer rice, winter wheat, sugarcane, and maize, farmers aim to reduce plant diseases and pests, eliminate weeds, replenish soil nutrients, and prepare fields for the next planting season. This method is also applied in vineyards and orchards for similar reasons. (2).

While crop burning offers immediate agricultural benefits, including pest control and soil renewal, it comes with substantial environmental and health costs. The process releases a number of pollutants into the atmosphere, including PM2.5 and PM10, which are fine and ultrafine particles capable of penetrating deep into the lungs and entering the bloodstream. Additionally, these fires release carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs) into the air. These substances contribute to the formation of ground-level ozone, a key component of smog, further exacerbating air quality issues.

Despite its localised nature of the burns themselves, the smoke can travel great distances, affecting air quality in regions far from the original burn sites, as vividly demonstrated by the deteriorating air quality in major cities such as Delhi. This global practice, therefore, has local actions with global consequences, emphasising the need for sustainable agricultural practices that mitigate harm to the environment and public health.

Where is crop burning practised?

Crop burning is widespread in some countries and minimally practised or banned in others.

Regulation and enforcement of crop burning laws varies strongly between countries, sometimes between neighboring states. In the European Union and the United Kingdom, there are strong restrictions against crop burning (3)(4). These restrictions sharply curb the amount of stubble burning that takes place across much of the continent, excluding non-EU countries like the Ukraine, Serbia, and Russia (5).

While these restrictions should mean clear skies across the continent, exceptions and the nature of transboundary air pollution can undermine health. Though agricultural burns are prohibited on paper in Russia, it is a widespread common practice (6). Two burning seasons – spring and fall - in the Kaliningrad enclave adjacent to Poland and Lithuania can regularly lead to polluted skies across that region.

Crop burning in Pakistan, northern India, Bangladesh, and Nepal can highly pollute the Indo-Gangetic Plain. The smoke is a primary contributing factor in some of the world’s most populous cities – Lahore, Dhaka, and Delhi – regularly being among most polluted major cities in the world.

Apart from South Asia, widespread agricultural burns regularly severely pollute the air in Southeast Asia, Sub-Saharan Africa, and northern South America (7). For example, despite restoration efforts by the government, Indonesian palm oil plantations burn peat marshes and peatland forests (8).

Burns are also used to create new farmlands in those regions by quickly clearing grasslands, rainforests, and peat marshes – “slash and burn” agriculture involving cutting down trees and plants, allowing them to dry, and then burning them.

Why is crop burning dangerous?

Crop burning is a common cause of poor health outcomes in Southeast Asia, South Asia, Latin America, and Africa due to the extremely widespread nature of the practice and accompanying clouds of far travelling smoke.

Smoke from agricultural burns carries PM2.5, particulate matter measuring 2.5 microns in diameter or less. It’s a pollutant that is so small that, when inhaled, it can enter the circulatory system and harm all parts of the body. Exposure to PM2.5 can lead to heart and lung disease.

Numerous studies have closely linked crop burning seasons to disease and a greater health burden. A study conducted between 2003 and 2019 and published in Nature Communications found crop burning and accompanying PM2.5 exposure led to between 44,000 and 98,000 premature deaths In India (9).

In 2023, two million Thais had to seek medical assistance due to air pollution, much of it linked to the country’s “burning season” between December and April (10).

Alternatives to crop burning

The considerable public health threat caused by crop burning has inspired a renewed look at these practices, with an emphasis on more sustainable, healthier alternatives.

As stubble removal by other methods like manual removal can be expensive, governments can take a proactive approach to providing speedier alternatives. Authorities in the Indian state of Punjab distributed 120,000 machines in 2022 to help destroy crop residue, while acknowledging that more will need to be distributed to help contain endemic crop burning (11).

Indian scientists at the Indian Agricultural Research Institute (IARI) have developed a “waste decomposer” designed to soften hard stubble and help convert the waste into compost (12). Composting improves soil quality, supports plant growth, and eliminates the smoke associated with crop burning.

Simply leaving standing stubble where it is can prevent soil erosion and helps with soil fertility. In Manitoba, Canada where straw waste is sometimes burned, the government recommends letting the waste lie during winter months to help improve winter wheat crop survival outcomes (13).

How can I protect myself from crop burning smoke?

Ending crop burning would improve the quality of life for millions around the globe.

In the meanwhile, the best way to avoid crop burning smoke exposure is to plan ahead and stay informed.

• Know when crop burning is commonly practised where you live and plan accordingly.
• When planning a trip, research to find out if your destination is affected by seasonal crop burning smoke when you intend to visit.
• Download a free air quality app for real-time air quality alerts and forecasts.
• Close doors and windows and set the HVAC to recirculate mode.
• If air quality is poor, don’t go outdoors. If you have to go outdoors, wear a KN95/FFP2 mask.
• Filter crop burning smoke by running a high-performance air purifier.
• Contribute to your community’s outdoor air quality data.