Every time contact lens wearers put in a new pair they create waste: the old pair of contact lenses, the packaging from the new pair and the bottle of saline solution. 

It all adds up to about a kilogram (2.2 pounds) per year, with reusable contacts generating slightly less. And with 140 million contact-wearers across the world, that means a lot of rubbish.

But eyeglasses come with their own set of problems. With about half of the world's population projected to need spectacles by 2050, working out which is better for the environment could have a profound effect.

Contact lenses create microplastics

About 20% of contact wearers in the United States flush their used lenses down the drain, according to Charles Rolsky. He is the executive director of the Shaw Institute, a US nonprofit which studies how contaminants, such as plastics, affect the environment and humans.

As part of his PhD thesis, Rolsky looked at the impact of waste from disposable contact lenses and found that somewhere between 2 and 3 billion plastic lenses end up in wastewater in the US alone.

He followed the journey of contact lenses through a wastewater treatment plant and looked at the plant's final product, a nutrient-rich fertilizer called biosolids.

The "really telling study" showed that the lenses survive the wastewater treatment process. "They're very porous. So, there's a chance that they could be contaminated with things like diseases or other types of chemicals, and they also became fragmented into microplastics," he said.

Microplastics are tiny particles of plastic that travel easily through the environment, especially in water. They can get into the food chain and eventually make their way back to humans.

A different study in 2023 found that at least 18 types of contact lenses sold in the US contain high levels of PFAS, otherwise known as forever chemicals. It is unclear whether the PFAS directly affect those wearing contact lenses, but these toxic chemicals can contaminate soil and water and build up in animals before potentially ending up inside humans.

So, glasses are better than contacts?

It's hard to say. Very few eyewear manufacturers in the $150-billion industry publicly report their carbon impact.

But once in the hands of consumers, glasses don't produce much waste, aside from the occasional cleaning wipe. The environmental impact largely occurs during manufacturing.

The lenses themselves are usually ground down from large pieces of plastic about the size of a hockey puck. Up to 90% of that original lump is cut away to form the lenses, according to Andrew Clark, a science communicator who helped found UK consultancy Net Zero Optics. Making the frames, which are mostly plastic, produces a similar amount of waste to the lenses. 

Manufacturing glasses is also problematic because of overproduction, according to Max Juraschek, a scientist at the Technical University of Braunschweig, Germany, who leads a group researching sustainable factory systems.

"Maybe half of them get actually thrown away before they can be sold, because it takes such a long time [from manufacture to final sale] and it's a fashionable product and maybe nobody's interested in this particular frame," he said.

Glasses are very much a fashion accessory, with many US wearers buying a new pair each year, according to Juraschek. Like other fast fashion items, the frames end up in landfill.

What about recycling glasses and contacts?

Glasses are produced using a complex array of materials including plastics that are difficult to recycle.

"We are such a plastic-heavy industry and the majority of it is derived from fossil fuels," said Clark. "We are in an industry that is very international, huge amounts of our manufacturing is done in China and over in the Global East." 

"Every step in that journey is either refining a plastic product or moving a plastic product. And it very quickly adds up to a substantial carbon footprint," he added.

In the UK, there are a handful of programs that claim to recycle both contact lenses and their packaging, as well as glasses. Contact lenses cannot be recycled alongside other plastic waste because they are so small and cannot be separated.

When it comes to glasses, specialty recycling programs try to separate them into their component materials before turning the plastics into low grade materials, which may eventually end up in landfill. 

Glass lenses are an alternative to plastic, but they are also difficult to recycle due to special coatings used.

What about sustainable glasses?

Frames are often made from acetate, a mixture of plant-based materials and fossil fuels. But glasses manufacturers are now marketing something called bio-acetate. This is simply greenwashing given how much plastic it still contains, according to Clark.

"I would liken it to creating a burger. You couldn't possibly get away with saying 'Oh, this is a vegan burger, 75% of it's vegan," he said. 

So, contacts or lenses?

To be mindful of waste is one of the most impactful decisions wearers of glasses or contact lenses can make. 

Contact wearers should avoid washing lenses down the sink — this just contaminates wastewater and the environment with microplastic. And if possible, finding a specialty recycling program could make a difference.

Glasses wearers can opt to only replace their lenses and avoid buying new frames purely for fashion. 

Juraschek's team found that by shifting production of glasses closer to consumers, and using local recycled materials, the environmental impact of glasses could be lowered by 25%.

Part of this success came from small-scale manufacturing that cut down on overproduction. The team also found customers had a greater connection to the product, as it was local. 

Edited by: Jennifer Collins

Alistair Walsh adapted this article from an episode of DW's Living Planet podcast. Find the audio version here.