Deep frying your dinner can contribute to air pollution

December 15, 2020

Food cooked in a deep-fat fryer. (Wine Dharma, Unsplash)

Inhalable particles from cooking can remain in the atmosphere for days and cause problems for air quality and human health much longer than previously known, new research shows.

The issue has yet to factor into environmental protection policies, despite the risks. Emissions from cooking food contribute up to nearly 40% of particulate pollution in some major cities, with deep-fat frying being especially problematic, according to research published in the Royal Society of Chemistry’s Faraday Discussions.

The hotter the oil used for cooking, the higher and farther the emitted particles can travel. On the journey, they have more time to absorb other harmful chemicals, such as carcinogenic polycyclic aromatic hydrocarbons that are emitted from vehicle engines, allowing them to be carried over much wider areas.

“Cooking emissions have been looked at for a while now, but people were surprised initially by how much cooking emissions contribute to particulate matters,” said Christian Pfrang of the University of Birmingham’s School of Geography, Earth and Environmental Sciences, and lead author of the paper published in September. “These cooking emissions can stay in the atmosphere for a very long time.” 

Usually, organic pollutant compounds are broken down quickly in the atmosphere by gases such as ozone. But by studying the process in laboratory conditions, the researchers found these emissions have properties allowing them to form a sort of crust that protects them from decay.

“It’s not the cooking emissions themselves that are the major health impact, but it’s what they hide underneath them,” Pfrang said. “It’s kind of a compounded effect together with the other chemicals that we emit into the atmosphere.”

At the moment, the emerging research isn’t really at play in public policy. There are regulations regarding how quickly people must ventilate commercial kitchens, for example, but nothing considering what happens once pollutants leave those buildings.

“If you think about emission control policies, you usually consider how fast and how far a pollutant can travel,” Pfrang said. “Obviously if this assumption is completely wrong — because you don't consider the effects we've described and we found — then this will have major implications in terms of the health protection of people living especially in urban environments.”

Sparked by other research providing greater understanding of exposure to indoor and outdoor air pollution, a new interdisciplinary network launching Tuesday seeks to bring the issue to the attention of city councils and local governments, especially as it impacts vulnerable groups such as children and people with lung conditions.

Academics from six universities in the U.K. and stakeholders there and in the U.S., China and Germany want to establish the key pollution hot spots and follow individuals throughout the day to assess their exposure, Pfrang said, ultimately providing “air pollution solutions” through technology such as ventilation but also behavioral interventions like avoiding events where pollution is high.

The group will assess the costs and benefits of implementing the interventions, and finally engage with businesses. The network includes around 30 stakeholders such as corporations that develop systems to improve air quality. Dyson, for example, has an indoor ventilation system.

Cooking emissions are an issue everywhere but can be especially harmful in large cities. They account for about 10% of small, inhalable PM2.5 particles in London, up to 22% in parts of China and 39% in Hong Kong, according to recent measurements. 

In the new research, Pfrang and his fellow researchers recreated the cooking process using a powerful X-ray beam at a large-scale facility. The technique allowed them to look at sub-microns much thinner than a human hair, observing at high resolution how fatty acid molecules arrange themselves to form very thin films on the emissions structures. 

Scientists then exposed the film to oxidants, which when found naturally in the atmosphere typically break down such molecules very quickly. But in this case, the film at a certain point formed an impenetrable crust, which Pfrang likened to an eggshell.

In this way, the researchers were able to follow the film’s decay, or rather, the lack thereof. By arranging themselves in this particular way, the molecules can become more persistent and stay in the atmosphere for longer, Pfrang explained. 

It’s already known that emissions vary between, for example, Chinese, Indian and European cuisine. Researchers are planning to study real cooking emissions from outdoors using the same method. The project was initially set for August but was delayed by COVID-19.

The study “The persistence of a proxy for cooking emissions in megacities: a kinetic study of the ozonolysis of self-assembled films by simultaneous small and wide angle X-ray scattering (SAXS/WAXS) and Raman microscopy,” published Sept. 7 in Faraday Discussions, was authored by Adam Milsom, University of Birmingham; Adam M. Squires, University of Bath; Ben Woden, University of Reading; Nicholas J. Terrill, Diamond Light Source; Andrew D. Ward, Central Laser Facility; and Christian Pfrang, University of Birmingham and University of Reading.

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