The shoe brand Converse is being accused of misleading people with its global public art campaign, Converse City Forests, in which it claims the murals it's painted are the equivalent of planting more than 9000 trees.
One of the murals was painted on Karaka Street in Auckland last month, and Converse claims the eye-catching 195 square metre artwork is the equivalent of planting 182 trees.
The paint used for the world wide project, made by Graphenstone, is photocatalytic, meaning a reaction can occur when UV light and a pollutant interact on the surface of the paint.
In a press release announcing the Auckland mural, Converse said:
"The mural has been installed with a unique photocatalytic paint that absorbs air pollutants as a plant would. The mural provides the equivalent of planting 182 trees.
"Globally, Converse has activated these sustainable murals from Singapore to Sao Paolo and planted the equivalent of 9,036 trees and counting."
And on the Converse City Forests website, they go further.
"We are partnering with our creative community to create Converse City Forests, a series of murals around the globe that uses Graphenstone's ecological air purifying paint to eliminate harmful substances such as carbon dioxide, formaldehydes and gases, improving the air quality around the murals.
"The paint's lime base absorbs CO2 during the drying process, acting similar to a tree. This way, any surface coated with it becomes an active air-purifying surface that helps protect people from harmful substances."
While some of the claims are true, if a little overstated, the claims about carbon dioxide are the most misleading.
Professor Justin Hodgkiss, co-director of the MacDiarmid Institute for Advanced Materials and Nanotechnology, and expert in photocatalysis said the paint does indeed absorb carbon dioxide, as Converse and Graphenstone claim.
But, it is a nett emitter of carbon dioxide, and he said to claim the murals are fulfilling the same role as a tree is disingenuous.
He said the paint is made using lime, and when it dries, the lime briefly absorbs CO2 and essentially turns into limestone.
"My understanding is that in order to put the lime into the paint, you would generally need to start from calcium carbonate, heat it up, which takes an enormous amount of energy, and drive off the CO2 that was on it to begin with to get the lime back.
"So you're actually emitting a lot of CO2 through the heat and through the cooking of the CO2 off the limestone, in order to make lime, and so it still has a nett emission of CO2."
Hodgkiss said the paint does not absorb enough CO2 during the drying process to make up for the CO2 used when the paint is made.
"For one litre of paint, which gives you about 12 square metres of surface area I think generally, that's about 350g of carbon, so we're talking about maybe 30 grams of CO2 per metre squared, which is not really an awful lot."
Graphenstone list the carbon footprint of their paint at 500 grams of CO2 per litre, meaning a nett emission of roughly 150 grams of CO2 per litre of paint used on the Converse murals.
That is far more carbon than 9000 trees would have produced.
RNZ asked Graphenstone to explain the claims the mural in Auckland was like planting 182 trees, and more than 9000 trees worth of murals had been painted around the globe.
Graphenstone's Market Lead, Jorge Fernández-Carvajal, sent RNZ a wealth of documentation on the paint's CO2 claims, which said 15 litres of its paint absorbs the same amount of CO2 as one, 250 kilogram tree does, during one year of growth.
That statement does not appear to take into account CO2 emitted during the making of the paint product, nor does it take into account that trees don't stop growing after just one year, and that they don't stop absorbing CO2 after just one year.
"This estimation is based on the chemical reaction that happens in a natural way (it does not need to be activated) when the paint dries," Fernández-Carvajal said via email.
"This drying process requires of CO2 to complete the carbonation process, as well as other acrylic paints just need the water on them to be evaporated."
The Graphenstone documentation also said the tree claims are solely to do with CO2 absorption, and no other properties of the tree.
Hodgkiss said some of the other claims of photocatalysis are legitimate, and the paint can indeed purify the air.
"Yes, that's a valid claim. Photocatalytic paints are known and they have materials called semiconductors in them which kind of react with light and they create charge which can react with things like volatile organic compounds or nitrous oxide, and convert them into less odorous things or things that are safer for the environment."
But again, some of the claims could be slightly misleading.
Hodgkiss said there is a big difference between the paint working in the lab, and on a mural in the centre of Auckland.
"Studies have shown that in a lab setting these photocatalytic surfaces do make a difference and perhaps in internal areas you would notice, maybe smells would disappear," he said.
"But studies have shown, in for example in the city of London, a photocatalytic pavement makes no measurable effect on the air quality versus the control background."
Hodgkiss said the messaging coming from Graphenstone and Converse is problematic.
He said the paint does appear to be a good product, and people should be encouraged to use products that reduce CO2, but the messaging around the Converse City Forests project has been unhelpful.
"The last thing we want is for people to think that painting their house with a certain paint absolves them of all of their guilt for behavioural choices and lifestyles that emit lots of CO2.
"People need to have a good grasp of all of the numbers, and what's making a real difference."
Converse could not be reached for comment.