Feeling under the weather? Read on to find out why

ALLERGIES

The worst weather for the allergy sufferer is often the best weather for the rest of us. "Sunny, dry, still days tend to have the highest concentration of pollen in the air," says Roy Harrison, professor of environmental health at the University of Birmingham. And it's not just hay-fever sufferers who are affected, he says, asthmatics react to pollen, too.

The bad news is that wind, which provides a welcome breeze for most, doesn't help. It can whip more pollen up from plants and carry it far. And while rain provides relief from stinging eyes and blocked noses because it washes out the pollen from the air, it is only temporary. "Post-thunderstorm is a particularly bad time because the storm can cause pollen grains to break up and explode, releasing lots of fragments into the air," explains Professor Harrison.

A warning for summer-loathing asthmatics - be careful what you wish for. Autumn, with its sudden colder temperatures, sees the largest number of hospital admissions for asthmatics as a result of the change in weather causing them to broncho-constrict, which hampers breathing.

HEADACHES

If you're susceptible to headaches or migraines, there's every chance you've been suffering over the last couple of weeks. One of the most common causes is a major change in the weather over a one- to two-day period, according to the latest study on the subject in Headache, a journal published on behalf of the American Headache Society. Another prevalent cause is either low temperature and humidity, or high temperature and humidity.

The study, conducted by Dr Patricia Prince and other headache specialists at The New England Center for Headache in Stamford, Connecticut, found that 51 per cent of patients with headaches were affected by weather. But before you smile smugly that you've always suspected this to be the case, take note that the study found a higher percentage of people thought they were affected than actually were. Nevertheless, Dr Prince says: "Identifying trigger factors, such as weather, is important as it can lead to preventative strategies such as trigger avoidance or taking acute- care medications very early in the attack or even in advance."

ACHES AND PAINS

If you already suffer from nuisance pain - especially as a result of arthritis, rheumatism or lower back-ache - you may find that certain weather conditions exacerbate the problem. The most likely scientific explanation is that membranes in the joints act as a barometer and expand as the air pressure drops. This can cause an increase in pressure of the fluids that lubricate the joints. This then offers more resistance to movement and increases the pains in those joints already compromised.

Damp, chilly, windy and humid weather are most commonly believed to be the worst enemies of pain sufferers, although Jun Sato, associate professor of the Research Institute of Environmental Medicine at Nagoya University in Japan, is unconvinced. "Given the many contradictory results and opinions in this field, I was struck by the absence of controlled animal studies addressing the issue," he says. So he did his own experiment with rats and found that pain was most likely to be aggravated by a mildly cold environment.

MOOD

Most people don't need telling that sunshine makes us feel better. We react to bright sunshine with optimism and happiness. Sunlight is thought to boost levels of serotonin, the chemical that produces feelings of wellbeing and elation. Other extremes of weather can also affect our moods: ensuing storms or fronts bring on restlessness, and mist bringing on feelings of anxiety. Scientists at the University of Louisville's School of Medicine in the US even found that incidences of low barometric pressure can be associated with an increase in violent behaviour. Seasonal Affective Disorder is the most well-known way in which weather can affect mood. The estimated half-a-million victims of SAD suffer from "winter blues" symptoms including low energy, problems with sleep and appetite, depression and reduced concentration. This is caused by a biochemical imbalance in the hypothalamus due to the shortening of daylight hours and lack of sunlight in the winter.

But, according to the psychologist Professor Alex Gardner, those who don't suffer from SAD can still feel pretty rough throughout the winter months due to the same chemical changes. "It's just that they bounce back more when the weather brightens," he says.

GOING INTO LABOUR

Beware of a fall in atmospheric pressure if you're expecting a baby. Hospital admissions of pregnant women reflect a steep rise in admissions during these times.

What's more, the more abrupt the fall in pressure, the greater the labour pains - with the inevitable consequence that birth often follows, even if the due date is a way off, particularly in regions with the lowest pressure or associated fronts.

Basically, the woman's body acts like a barometer - the womb is a fluid sac that expands and contracts with changes in atmospheric pressure. When atmospheric pressures drop rapidly, the womb expands and exerts additional pressure, possibly triggering labour.

MENTAL ALERTNESS

Do you find your brain is brighter or foggier than usual in some weathers? Scientists recently asked 12 volunteers to perform several mental exercises, from proofreading to memorisation, while researchers varied the barometric pressure in the room. Even small, controlled changes in pressure made the alert volunteers perform better and the sleepy subjects perform worse.

But, when the researchers - who published their findings in the International Journal of Biometeorology - varied the pressure randomly, to synthesise the conditions during stormy weather, all the subjects experienced concentration lapses. Although nobody is quite sure why this happens, one possible explanation is that air pressure may cause changes in blood pressure, affecting brain activity.

Employers should take note - quality of work and levels of productivity can diminish when the weather is changeable. Likewise, schoolteachers should be aware that disciplinary problems can increase during variable conditions. Sportspeople and drivers can be adversely affected, too.

EXHAUSTION

While sunshine is needed by our bodies to make vitamin D, heat is the biggest killer by far when it comes to weather-related deaths, says Dr Laurence Kalkstein, president of the International Society of Biometeorology. "This is something that was unrecognised as little as five years ago," he says.

Quite simply, it can cause heat exhaustion, which is why people often feel lethargic at this time of year. For weather-sensitive individuals, this can bring on heart attacks, strokes and respiratory failure. "In the 2003 heat wave in Europe, we know that at least 35,000 people died," he says. "We also know that in the States - where we are based - heat kills more people than all the other weather killers combined."

He adds that when Hurricane Katrina hit New Orleans, 1,300 people died, but in an average summer in the States, there are 1,500 heat-related deaths. Heat exhaustion is most likely to hit when weather is variable, he says. "That's why you get few deaths in Florida, where the weather is the same every day, even though it's hot. But where you get a few mild days, followed by a string of days with high temperatures, the alarm bells should ring."

Illness and the atmosphere

* One in three people are estimated to be "weather sensitive". Women are more susceptible than men, and the very young and very old are also particularly likely to have their health and wellbeing affected by the weather.

* Hippocrates was the first to write about the connection between weather and health in 400BC. His ideas were developed in the Middle Ages, when herbalists prescribed specific plants for different winds. The rise of empirical science led to these ideas being rubbished, but centuries later scientific experiments led to a revived interest in the effect of weather on health.

* Climatotherapy is the latest must-have for the wealthy unwell in the States. The idea is that different weather conditions treat different illnesses. In fact, the concept isn't new - for example, seaside climates have been recognised as good for bronchitis and rheumatism.

* Beware of spending too much time in artificial climates with ionisers, air conditioning or humidifiers. Bad air-conditioning systems have been blamed for Sick Building Syndrome, where increased concentrations of bacteria and other pollutants such as cigarette smoke can cause health complaints.

* In Germany, where biometeorology (the effect of weather on health) originated in the 1940s, daily weather forecasts include information on what health problems may be in store in certain regional areas.

* If you're using a fan to cool down in hot weather, make sure you drink plenty of fluids. "A fan draws moisture from the body," says Dr Laurence Kalkstein, president of the International Society of Biometeorology. "We've had cases of people being found dead from dehydration in front of them."

* Avoid very cold water in extreme heat. Jumping in a very cool pool or bracing shower can cause blood vessels to keep the blood away from the skin and retains heat rather than cooling you down.

* It's a myth that you can get a cold from cold and damp weather. Paul Hunter, professor of health protection at the University of East Anglia, says: "It's just that we tend to stay indoors more during this weather, where germs spread more easily."

* Between 10-15 minutes of sunshine goes a long way, say experts, whose message is that too little of it leaves us deficient in a vital nutrient, vitamin D. The move follows a change of policy in Australia and New Zealand, where scientists have decided that, without some sun on the skin, the population will be seriously deficient in vitamin D and may be at risk of developing cancers later in life.