Watch the report by Rob Osborne
There are calls to crack down on the sale of nitrous oxide after one litter picker told ITV News he was "shocked" to find more than 700 discarded canisters in one park.
Dave King told ITV News he filled two bags full of the nitrous oxide canisters and counted a total of 726 in Canal Park in Butetown, Cardiff.
Nitrous oxide gas - also known as "hippy crack" - is a popular drug amongst young people.
Dave is part of Keep Grangetown Tidy and the Cardiff Rivers Group and said he has seen more of the metal canisters since lockdown restrictions were eased.
He said: "The main thing I came across was literally hundreds of the nitrous oxide gas canisters. There must have been dozens of people sitting around inhaling the gas and dropping the canisters where they were.
"When I got home, I thought I would count them up because there were so many and it came to 726 in just one park."
"During lockdown we didn't see very many at all, but then suddenly it was almost as if a new supply had hit the streets. There was a huge increase and we were just seeing them everywhere.
"They are all over the place, you see them all over the road and in the parks particularly."
What is nitrous oxide?
Nitrous oxide it traditionally used in baking or in hospitals as a painkiller.
It is not illegal to carry but it is against the law to supply it for human consumption - and can lead to up to seven years in prison.
Nitrous oxide is most commonly used by those aged between 16 and 24 years old.
A recent Global Drug Survey listed it as the fourth most used drug in the UK, behind ecstasy, cocaine and cannabis.
The gas causes short-lived feelings of euphoria and sends people into fits of giggles when inhaled, but it can lead to fainting or suffocating because of a lack of oxygen intake.
Other risks include reckless or dangerous behaviour, and vitamin B12 deficiency.
The Royal College of Nursing claims nitrous oxide is often seen as being low risk and offering a short, harmless burst of euphoria. But the Office for National Statistics data shows there were 25 deaths directly linked to the gas between 2010 and 2016.
Dave King has been helping to keep Cardiff clean for 17 years and was awarded an MBE for services to the environment.
He says it is disappointing to see huge amounts of rubbish left behind after large crowd gathering in places like Cardiff Bay.
"I hate to see it. Cardiff is such a great place, we're lucky with so much greenery. It's got the rivers, the parks, everything. And then it's just spoilt by the litter."
He continued: "I think we had an opportunity to be doing things differently and appreciate what we've got after everyone being locked down.
"I'm quite disappointed that people have gone the complete opposite way and think it's okay to trash everything even worse than they were before.