The clocks have gone forward, the evenings are growing lighter and across the land, GP surgeries are bracing themselves for an influx of sneezing, snivelling patients begging for help with their hay fever. About one in four Britons, or 16 million people, are now affected by allergic rhinitis – to give hay fever its official name – compared with just one in eight in the early 1980s. A century ago, the illness was almost unheard of.

No one is sure about the reasons behind this astronomic rise, but the majority of immunologists point the finger at the hygiene hypothesis: the theory that our ultra-clean homes have left many immune systems less able to tolerate irritants. Yet despite – or maybe because of – hay fever being so widespread, many people are ignorant of quite how debilitating it can be. “Family members, GPs, even patients themselves can dismiss hay fever as just a bit of sneezing, but for about 10% of sufferers it causes abject misery,” says Professor Stephen Durham of the Royal Brompton allergy clinic in London. “It worst affects young, active people at work and school: studies show it can increase by 70% the chance of their summer exam grade being worse than their previous, out-of-season mock test results.”

But young people aren’t the only ones suffering: studies show that increasing numbers of adults in their 30s, 40s and 50s are developing hay fever. Half a million new “middle-aged” cases are predicted in the next decade, according to the National Pollen and Aerobiology Research Unit. Of these, many never even suspect an allergy, instead believing they are suffering from a permanent cold. Dr Adrian Morris, the director of the Surrey Allergy Clinic, says: “Many go to the GP complaining of sinus problems and end up on antibiotics, when they really have hay fever and need antihistamines and nasal sprays.”

Others, warns Amena Warner, a nurse adviser at Allergy UK, treat their stuffiness with over-the-counter decongestants, which, used long-term, leave the nasal lining even more inflamed and susceptible to problems. “You need a correct diagnosis so you can properly treat the problem,” she says.

There are also many people convinced that they have hay fever when in fact they are suffering from a different allergy. “Hay fever is a stupid name, really,” says Durham. “It is an allergy to grass, not hay, and it doesn’t produce a fever.”

If you are sneezing and have itchy eyes before the grass pollen season starts in June, you may have an allergy to birch pollen, which is increasingly common. Other trees and plants that spark allergies at different times of year include plane, oilseed rape, oak and nettle. If your nose becomes runny in early spring or autumn, it could be the result of a mould allergy. If the sniffing continues all year round, you might have an allergy to dust mites, mould or pets. “You may be attributing your sneezing to pollen, when in fact it is because you are sitting next to someone in the office who has a cat,” says Morris. What it is not so likely to be is a food allergy: these affect only 3% of the population, although 30% believe they have one.

Those in doubt should be referred to an allergy clinic, where tests, involving pricking your skin with tiny amounts of allergens to check for reactions, can be carried out. But waiting lists are often months long: Allergy UK points out that there are only 30 allergy specialists in the UK, one for every 700,000 sufferers.

And testing doesn’t always provide answers, as I discovered myself. After years of a permanent runny nose, blocked ears and volcanic sneezes, my GP finally referred me to a clinic. My skin-prick tests all showed negative and I learned I was suffering from non-allergic rhinitis, meaning I have all the symptoms of hay fever, but no identifiable allergy.

I’m far from alone: one Norwegian study estimated that as many as 25% of the population suffers from this condition, which can make life every bit as miserable as for those with a known allergy. Experts usually link the condition to changes in temperature or hormones, or sensitivity to environmental factors such as car exhaust, perfume or detergent.

But to make matters even more confusing, current research indicates that as many as 50% of those with non-allergic rhinitis may actually have an allergy after all, but one that skin-prick tests can’t identify. Until new, more sensitive tests appear, millions of us may remain in the dark. And without knowing exactly what our allergy is, tackling symptoms can be difficult. Most allergic rhinitis symptoms can be controlled with over-the-counter antihistamines such as loratadine that, unlike first-generation pills, don’t make you drowsy. Non-allergic rhinitis sufferers, however, are better off with a nasal spray such as Beconase.

Humidifiers, such as Dyson’s latest device that mists the room with ultraviolet-treated, bacteria-free water, can hugely relieve stuffy noses and sore throats. Conversely, they can produce ideal breeding conditions for dust mites and mould. Similarly, drying sheets in sunshine helps those allergic to dust mites, but is a bad idea for people with hay fever as it coats bed linen in pollen.

It is crucial, then, to know exactly what your enemy is. “It is very easy to misdiagnose allergies – especially with the help of Dr Google – and therefore for severe cases, it is a good idea to get a referral to an NHS allergy clinic,” says Durham. But with those lengthy waiting lists, in the meantime remember to stock up with tissues.

What’s your allergy?

Birch: Second week of March to first week of June (peaks last week of March to mid-May).

Plane: Mid-March to mid-May (peaks last week of April to second week in May).

Oilseed rape: Last week in March to mid-July (peaks mid-May to end of June).

Oak: First week of April to mid-June (peaks end of April to first week in June).

Grass: First week of May to second week of September (peaks first week in June to last week in July).

Nettle: Beginning of May to end of September (peaks last week of June to first week in August).

Mould: Early autumn and late spring. Symptoms are worse inside than outside.

Dust: All year, but especially notable in winter when the central heating is turned on. Symptoms are worse indoors.