9 Aug 2019 - BBC - Plastics, arsenic, lead and nicotine from butts can all harm marine life. Just one per litre of water can be highly toxic to fish, research suggests.
Cigarette butts - flicked into drains, mashed under foot or dropped in parks - are a common sight around the world.
But a campaign launched by a group of French teenagers to tidy them up has highlighted their environmental impact.
Amel Talha launched the hashtag #FillTheBottle after a friend collected cigarette butts in a water bottle and posted a photo on Twitter.
The campaign has inspired thousands to clean up what is thought to be the most common form of litter around the globe.
"This is a big problem in France but also all around the world," the 18-year-old tells the BBC, adding she is "extremely happy and proud" that the campaign has had such an effect.
Amel's friend Jason Prince first posted an image of a plastic water bottle filled to the cap with cigarette butts last Wednesday.
"20 minutes to fill this 1 litre bottle in an area of less than 50m," he wrote. "This is extremely serious."