Allergenic Pollen Types

Hazel

Latin name: Corylus spp.

Plant description: Small, bushy tree with oval, serrated leaves. Pale yellow catkins produce the pollen. Fruit: hazelnut/filbert.

Distribution: UK: Countrywide. Also across much of Europe, North America and Asia and planted as an ornamental in more temperate regions.

Allergenicity and other health impacts: Hazel pollen has moderate to high allergenicity. It can cross-react with birch pollen so some birch-pollen sufferers may be affected by hazel pollen. There are only two allergenic pollen types that occur in the winter, hazel and alder, so if you have symptoms at that time then one or both of these may be the culprit.

Season: Mid-January to mid-April, usually peaking in February.  The peak can start as early as December in very mild Winters or be delayed if the Winter weather is severe.

Alder

Latin name: Alnus spp.

Plant description: Medium sized tree with oval, serrated leaves, the tip indented on some species. Dense, wine-red catkins produce the pollen. Fruit: small cones, green at first, then brown.

Distribution: UK: countrywide. Also across much of Europe, Western Asia, North-Western USA and North Africa, growing in or near watercourses.

Allergenicity and other health impacts: Alder pollen has moderate to high allergenicity. It can cross-react with birch pollen so some birch-pollen sufferers may be affected by alder pollen. There are only two allergenic pollen types that occur in the late winter, hazel and alder, so if you have symptoms at that time then one or both of these may be the culprit.

Season: Mid-January to mid-April, usually peaking in February but the peak can be delayed if the Winter weather is severe. 

Birch

Latin name: Betula spp.

Plant description: Medium sized trees with small, triangular, serrated leaves and white bark. Yellow catkins produce the pollen. Fruit: composed of layers of small seeds in a catkin shape, green at first, then brown.

Distribution: UK: countrywide. Also across much of Europe, Asia and Northern USA, restricted to cooler climates or found at altitude.

Allergenicity and other health impacts: Birch pollen affects about 25% of the UK’s hay fever sufferers and has high allergenicity. It can cross-react with hazel and alder pollen and some tree-pollen sufferers can experience symptoms starting in Winter and lasting until late May or early June.

Season: Mid-March to early June, although the start of the season can vary by up to one month and often occurs about two weeks later in Scotland compared to southern England.

The peak period occurs in April. Birch trees each produce millions of wind-dispersed pollen grains so in the peak of the season the count can be very high.

Oil seed rape

Latin name: Brassica napus

Plant description: Oil seed rape is a crop grown for vegetable oil. The bright yellow fields with their distinctive scent have become 
a familiar site in the UK countryside.

Distribution: UK: arable areas, especially the south, Midlands, East Anglia and some parts of Northern England and southern Scotland. Also across much of Europe to the Mediterranean.

Allergenicity and other health impacts: The crops are often blamed for triggering symptoms of hay fever although only around 1 in 25 sufferers of seasonal allergies test positive to oil-seed rape allergen. In addition, the flowers are insect-pollinated and only low to moderate amounts are caught on the wind and dispersed away from the crop. The oil-seed rape flowering season coincides with those of the more highly allergenic birch and oak pollen seasons which are far more likely to cause symptoms. However, the crops do give off volatile organic compounds which can cause irritation of the upper respiratory system, giving a range of symptoms from irritated eyes to coughs and even bronchial conditions in some people.

Season: Late March to early July, usually peaking in April or early May.

Plane

Latin name: Platanus spp.

Plant description: Large tree with large, 5-pointed palmate leaves and grey/green mottled bark. Green, round flower-heads produce the pollen. Fruit: reddish-brown, finely hairy, round ball.

Distribution: UK: often planted as a street tree and widely planted in parks and large gardens, particularly in London and other parts of south and central England. Also across much of Europe, Asia and Northern USA.

Allergenicity and other health impacts: Plane pollen has high allergenicity while the hairs from the fruits can induce asthma.

Season: Plane pollen can start in late March and end around late May but the peak of the season usually occurs in late April and early May. The counts near the trees can be very high but the pollen does not spread very far so where plane trees are not planted (e.g. most of Northern England) plane tree pollen is unlikely to be present in the air.

Oak

Latin name: Quercus spp.

Plant description: Large trees with lobed leaves and grey bark with knobbly ridges. Groups of greenish-yellow catkins produce the pollen. Fruit: acorn. There are some evergreen species of oak with smaller, thick, smooth-edged leaves.

Distribution: UK: countrywide. Oak trees are native to Europe, Asia and the Americas.

Allergenicity and other health impacts: Approximately 20% of hay fever sufferers are allergic to oak pollen which has moderate allergenicity.

Season: April and May to early June, although the start of the season can vary by up to one month and often occurs about two weeks later in Scotland than in the rest of the UK. The peak of the season occurs in April in early seasons or in May in later ones.

Grass

Latin name: Poaceae (family)

Plant description: Long, slender leaves with green, spikey flower-heads (pannicles). Grasses are usually between 5 – 100 cms in height when in full flower. The pollen is produced on the tiny anthers which hang down from the pannicles. The anthers may be yellow, white, green or purplish in colour.

Distribution: worldwide distribution. Approximately 150 different species occur in the UK (not including ornamental grass species).

Allergenicity and other health impacts: Highly allergenic, affecting approximately 95% of hay fever sufferers and can also induce asthma attacks. Grass can produce a rash if brushed on bare skin and the sap can become airborne and induce allergy (e.g. when mowing the lawn).

Season: Depending on the weather, the grass pollen season can start in April or May with low levels initially. The south of the country can start much earlier than the north. Moderate and high levels don’t usually occur until late May or early June. The peak season is usually in mid-June and the season usually goes into decline in mid-July but there can be grass pollen airborne until the beginning of September.

Daily pattern of release: Grass pollen levels are highest on warm, dry days. On these days, the pollen is at its worst in the first half of the morning and again from late afternoon and into the evening.

Nettle family

Latin name: Urticaceae (family)

Plant description: There are several members of this family but the two that are most important to allergy sufferers are common nettles (Urtica dioica) and pellitory-of-the-wall (Parietaria judaica). They have very similar pollen borne on tiny flowers which are greenish in nettle and reddish/white in pellitory-of-the-wall.

Distribution: Pellitory-of-the-wall: In the UK, S. & E. England, Midlands, coastal Wales, very local in N. England, rare in Scotland, also found in Ireland. Grows in crevices of old walls. Nettles: Widespread. Rest of world: nettles are cosmopolitan in temperate climates, Pellitory of the wall is widespread in much of C. & S. Europe (particularly the Mediterranean) and the Americas.

Allergenicity and other health impacts: Generally moderate allergenicity in the UK. The nettle family are classed as ‘Weed pollen’ for forecasting purposes. It is not known exactly how many people in the UK are affected by nettle pollen as very little work has been done on this topic. However, a study conducted in Southampton[1] found that out of 62 patients that were skin-prick tested for sensitivity to a range of allergens, 13 were sensitive to nettle and 8 to pellitory-of-the-wall.  

Season: June to early September, with peaks in late June/early July and again in mid-August. There tends to be a daily peak around 6pm in the evening on high count days[2]. Nettle pollen is released in the highest quantities on warm, dry days with a gentle breeze.

Plantain, Ribwort

Latin name: Plantago spp.

Plant description: ‘Weed’ with either long, lance-shaped leaves (e.g. P. lanceolata) or shorter elliptical leaves (e.g P. major). The flower-head is borne on a stalk and is dark brown and barrel-shaped with the pollen-producing, creamy-coloured anthers standing proud.

Distribution: UK: widespread in grassland, roadside verges and cultivated ground. Rest of World: extends from northern Europe through to southern Europe and into northern and central Asia, temperate parts of the USA, Australia, New Zealand and South America.

Allergenicity and other health impacts: Most members of the family contain allergenic proteins, causing seasonal allergic rhinitis and triggering asthma, but English Plantain (Plantago lanceolata) is the most highly allergenic.

Season: April to August. Concentrations tend to be low in the general airstream but higher near to the actual plants. Plantain is wind-pollinated.

Dock

Latin name: Rumex spp.

Plant description: ‘Weeds’ with large, elliptical leaves and a tall flower stalk. The flowers are green to reddish and inconspicuous and carried on long spikes. The seeds are small, brown and shiny.

Distribution: Dock is an invasive weed which has spread from its native Europe across much of the world. It is largely found on waste ground, waysides and cultivated ground but occurs in many types of habitat.

Allergenicity and other health impacts: Dock pollen contains allergenic proteins, causing seasonal allergies, but it is not known how many people in the UK are affected.

Season: May to September. Dock is wind-pollinated

Mugwort

Latin name: Artemisia spp.

Plant description: Tall, many-branched ‘weeds’ with a reddish stem and fern-like leaves that are downy and white beneath. The flowers are small, greenish and inconspicuous and carried on long spikes.

Distribution: widespread with representatives in temperate regions of the world, preferring dry or semi-dry habitats. It is found on disturbed ground, waysides and field margins.

Allergenicity and other health impacts: Mugwort is quite highly allergenic and can cross-react with many foods, including peach, lettuce and celery. It can also cross-react with other pollen types from the daisy family, including the highly allergenic ragweed which is common in central Europe and the USA.

Season: Late June to early September, peaking in July. Mugwort is wind-pollinated but concentrations of the pollen tend to be low in the general airstream and higher near the actual plants