Your eyesight may be rounded down to fit the budget

Personal carbon budgets have mostly been discussed in terms of flights, meat and the boiler. Under a scheme being modelled for high-street opticians, they would also cover the lenses in front of your eyes. The prescription would remain accurate; it would simply be offered in an affordable version.

Personal carbon allowances, in which individuals or households receive a tradable yearly emissions budget, have been debated by academics and policymakers for two decades, and several banks and apps now show customers an estimated carbon footprint for their spending. Short-sightedness is rising sharply worldwide, particularly among children, and time spent outdoors is one of the few interventions shown to slow its onset. The optical industry faces growing scrutiny over the waste from daily disposable contact lenses and their packaging, and over the footprint of lens manufacturing and distribution. Health systems in several countries have adopted net-zero targets that extend to the products they prescribe.

Agent: I can see your prescription has moved by minus 0.50, which takes the household over this year's optical allowance. Customer: Can I borrow some from my mother? She barely reads. The exchange, from a training transcript for chat staff, would become routine by 2030 under a scheme in which every eye test returns two results: what you can see, and what you can afford to correct. A national carbon budget, already covering heating, travel and food, would be extended to optical products, with each dioptre of correction assigned a footprint based on resin, coatings and the delivery van. The patient would leave with a prescription and a balance.

The test itself would not change. The letters would still shrink, the optometrist would still ask whether it was better with one or with two, and the patient would still guess. What would change is the printout. Beside each figure would sit its carbon cost, calculated by a new body, the Optical Allowance Authority, from the lens material, the thickness at the edge and the number of pairs a person of that age is statistically expected to lose. A strong prescription would cost more to see through than a weak one. Astigmatism would carry a surcharge. The leaflet would describe this as a fairness measure.

Patients over their allowance would not be refused glasses. They would be offered what the scheme documents call a budget-aligned correction: a prescription rounded to the nearest quarter dioptre in whichever direction made the lens thinner. The optometrist would demonstrate the difference on the chart, and the patient would sign to confirm they had seen it, or most of it. “He said I'd still be legal to drive,” said Dorota Wiley, 44, a payroll clerk who would receive a budget-aligned pair under the pilot. “He also said not to drive at night, or in rain, or towards anything.”

Daily disposable contact lenses would fare worst. Each pair, with its blister, its foil and the box the blisters arrive in, would be scored individually, and a wearer of thirty years' standing would be shown a running total on the patient portal, in kilograms. Monthly lenses would be encouraged. Annual lenses, withdrawn from most markets on grounds of hygiene, would return with a cleaning rota and a consent form. “I've done the maths,” said Kemal Osman, 29, a bar manager. “Either I wear glasses for the rest of my life or I join the waiting list for laser.” The waiting list for laser would be measured in carbon too.

Children would be handled separately. Because short-sightedness in the young is known to be slowed by time outdoors, schools would receive a daylight allocation instead of an optical one, to be spent in the playground. A child whose prescription worsened faster than the regional average would trigger a review of the household's screen hours, which would already be on file. Parents would be invited to a meeting. “They showed us a graph of his eyes and a graph of the tablet,” said Priya Lindqvist, whose son would be eight. “The lines matched. Nobody said anything. We all just sat there looking at them.”

A spokesperson for the Authority would describe the allowance as “a nudge, not a limit”, and would stress that nobody would be denied a clinically necessary correction, only invited to reflect on whether they needed the whole of it. Trading would be permitted. Households with good eyes could sell surplus allowance to households with bad ones, at a price set by the market. Early modelling would suggest that surplus would accumulate among the young, the outdoorsy and people who do not read, and would be bought up by everyone who does.

The chart would be the last thing to change. In the scheme's second year the bottom row of letters, which few patients can read and the scheme would no longer subsidise, would be removed from the standard test, and the row above it would become the bottom. Retailers would call this a simplification. The trade body would call it a recalibration. Mrs Wiley, asked whether she had noticed anything different about the world since collecting her new pair, would think for a moment. “It's softer,” she would say. “Everything is a bit further away than it was. I've stopped waving at people, in case.”