This alarming statistic is the basis of a new campaign from the UK's biggest blindness charity. The RNIB wants the Government to fund a major public education drive on the subject of glaucoma to alert the public to its dangers and effective treatments.
The campaign, Tunnel Vision, is supported by broadcaster Sir Trevor McDonald and writer and comedian Jenny Eclair. It was officially launched at the House of Commons last week.
In April, the RNIB commissioned Taylor Nelson Sofres to conduct a survey on the prevalence of glaucoma throughout the UK. The market research agency sampled 1,349 adults - from the results, the charity estimates that more than half a million people have glaucoma and 216,000 have serious sight problems as a result of the condition.
At last week's campaign launch, the RNIB unveiled its report based on the research. Tunnel Vision reveals huge ignorance of glaucoma - one in five people have never even heard of it. Just 2 per cent of those aware of it know it could lead to 'tunnel vision' and just 3 per cent know it might have no symptoms.
The RNIB wants to increase the public's knowledge of glaucoma and drive home the message that regular eye tests can detect the condition in its early stages and potentially save someone's sight.
Those who already have glaucoma and people considered to be at risk are eligible for free eye tests.
As well as lobbying the Government, the RNIB will host a series of nationwide seminars for health and social care professionals. It wants to stress the importance of alerting their clients to the risks of glaucoma and the availability of free eye tests.
The charity is also planning to connect directly with the public through a three-month, 13-city roadshow starting in Bristol on Friday. Exhibition stands staffed by RNIB experts will be put up in shopping malls, and shoppers will be invited to take part in an interactive quiz voiced by Sir Trevor McDonald.
Eye tests will not be carried out on site, but shoppers deemed to be at high risk of glaucoma will be encouraged to see an eye specialist.
Steve Winyard, the RNIB's head of public policy, said: "People need to be aware that glaucoma does not necessarily have any symptoms, and once you have the symptoms it's too late.
"An eye test can save your sight. This is even more important to people over 40 and those with a close relative with glaucoma - they are at higher risk. And people of African origin are four times more likely to develop glaucoma than the general population."