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The canisters have become part of fan celebrations in several countries, and are likely to make more appearances as the international soccer tournament continues in the coming weeks. Here are some key things to know about them:

What are smoke canisters and why do fans use them?

Colorful smoke bombs are a popular way for soccer supporters to celebrate and create a match-day atmosphere.

Greg Yerashotis, an assistant professor of sport, gender, health and social inclusion at Trent University, said this type of fan expression can originally be linked to superfans known as "ultras"— a portion of a team's fanbase that consists of its most extreme and passionate supporters.

This type of fanbase has existed for at least a century, originating in Italy, and the tradition has since spread worldwide, Yerashotis said.

Theatrics are central to the fandoms, he said.

A smoke bomb "signals that something is about to happen," he said. "It's an anticipatory thing. There's a mystery and a mystique. It's a way of colour coding your support. These fans identify through their sport allegiances, and they stylize and want to express that identity."

Why are smoke canisters banned in stadiums?

In stadium settings, flares can obstruct sightlines for security personnel, making it difficult to monitor crowds or respond to incidents, Yerashotis said.

Professional stadiums across the world have experienced dangerous incidents tied to pyrotechnics, which is why they are tightly regulated and strictly prohibited in most venues, including all World Cup stadiums.

What impact have stadium bans had on their use?

Enforcement can be difficult when large groups of supporters gather outside stadium grounds, and bans have driven increased outdoor use among supporters, said Yerashotis.

"Whenever you ban something, it kind of grows because there's an intrigue," Yerashotis said. "Because of its banning inside the stadiums, we're going to get more of that use outside the stadiums."

Are there health concerns related to the use of smoke canisters?

Smoke bombs release thick clouds of particles that scatter light, producing the vivid colours seen at matches, according to Greg Evans, director of the Southern Ontario Center for Atmospheric Aerosol Research at the University of Toronto.

The chemical makeup differs per product, but copper and antimony are key components in some coloured pyrotechnics. Copper is a particular concern since it is monitored in other health contexts, Evans said.

Research on their health effects remains limited, but studies using lung cells have shown the smoke bomb particles trigger inflammation, Evans said. Animal testing has produced similar results, with effects typically manifesting as respiratory issues in the short term, he said.

"There's no question that there's huge concentrations of particles being emitted," Evans said. Depending on wind conditions, people will inhale these particles for anywhere from a minute or two to much longer.

Vulnerability to these particles varies significantly. Young children whose lungs are still developing, elderly individuals, and those with pre-existing conditions like asthma face heightened risk. But Evans noted the range of vulnerability extends across a large segment of any stadium crowd or street march.

This report by ºÃÉ«tvwas first published June 24, 2023.