About half of all cases of hayfever are caused by airborne allergens. The most common of this type of allergen is pollen. All flowering plants produce pollen as part of their reproductive process. It is a means of transporting the male sex cells of the flowering plant to the stigma, the moist female organ of a plant. The exterior of pollen granules contains around 40 different types of protein. This is to enable the female stigma to recognise the correct pollen for pollination. It is these proteins that prompt the immunoglobulin (IgE) antibodies of the body's immune system to react when they are inhaled or come into contact with the membranes of the eye. The IgE antibodies bind to mast cells and trigger the release of chemicals, including histamine, in an attempt to combat these foreign proteins which are mistakenly seen as a threat. Pollen granules are very small, being measured in the microns. When viewed under a microscope, most pollen granules appear as intricate, roughly spherical structures. A mass of pollen can be seen with the naked eye but each individual pollen granule would be too small to see on its own. Single pollen particles are usually too large to pass the nasal mucosa and reach the lungs but in certain atmospheric conditions water particles can actually cleave the pollen and break off micro-aerosols that carry allergens small enough to be inhaled into the lungs. Plants often rely on visiting insects to transfer pollen, hence the bright colours and distinctive aromas of their flowers. Such colourful or flagrant plants rarely cause allergies and are not usually a factor in hayfever because their pollen is too heavy to become airborne. Their pollen is generally larger and covered in a waxy, sticky coating to make it easier for insects like bees and butterflies to collect. Some individuals can develop allergic symptoms in response to the aroma of these sorts of flowers, including wheezing or sneezing, but they usually need to come into prolonged, close contact with the plant to be affected. People in occupations like gardener and florist are most likely to become sensitised to the pollen from these types of plants. The plants that cause problems for hayfever sufferers are those that use wind pollination. This category includes trees, shrubs, weeds and grasses. These plants are usually plain looking because they do not need showy flowers to attract insects. The wind catches the pollen in the dangling anthers of these plants, which are the pollen-bearing part of the stamen, and carries it through the air in the hope that some of it will land on a receptive stigma. It is a very hit-and-miss affair that requires large amount of pollen to be released to have any chance of success. Some plants produce many millions of pollen granules every season, much the chagrin of those who suffer from hayfever. To give you an idea of how far this pollen can travel, samples of ragweed pollen have been collected 400 miles out to sea and two miles high. Because of this ability to travel, hayfever may be triggered by plants that do not even grow in your geographical region. While pollen is the major cause of hayfever, the type of pollen that commonly serves as a trigger differs from country to country. For example, in America it is ragweed pollen that is prevalent. In Japan, however, the main hayfever culprit is cedar pollen, while in Scandinavia it is the pollen of the birch tree. The Japanese also use the silver birch as a popular type of ornamental deciduous tree but this practice has inadvertently resulted in more than 30% of Japan's population becoming allergic to its pollen. During seasons when plants are pollinating, people with allergic tendencies may develop sensitivity to one or more types of pollen. The length of time that a plant produces pollen depends on the geographical region, the climate, and the hours of daylight that the plant is exposed to. Pollen concentrations will also vary depending the time of day and weather. Favourable conditions include warm, dry windy days with higher concentrations of pollen usually being encountered from morning until noon. The "pollen count" is a measure of the amount of pollen in the air for that day so that allergy sufferers have a general measure of when it is best to stay indoors. This represents the concentration of pollen in the air in a certain area at a specific time, measured in granules of pollen per cubic metre of air collected during a 24-hour period. There is also a "spore count" for those with mould allergies. The pollen count will tend to be at its highest early in the morning on warm, dry, breezy days and at its lowest during cold, wet days. This can vary greatly for one type of allergen compared to another. In the US for instance, one autumn day in 2001 had a ragweed count of only three granules of ragweed per cubic metre while the mould spore count on the same day was a staggering 15,000 spores per cubic metre.