• Pollens are allergenic factors:
  - the pollens of small size (less than 10 microns) enter in respiratory tract and lungs. The bigger pollens of 20 and 60 microns are less dangerous because they are stopped in the upper respiratory tract.
  Besides, the small pollens are light and more disseminated by the wind than the heavy pollens. For this reason allergenic pollens will be found as well in the cities as in the campaign.
  - only the pollens which liberate sensitive protein particles are responsible for allergies.
  A sensitive person has a "genetic background" : the atopic background.
• Which trees are likely to cause allergy attack:
  anemophilous trees, that is those which disperse in air the grains of male pollen in search of female pollen. The risk of allergy due to the pollens of trees is therefore linked to the season of the pollination, in spring. The risk of allergy is the highest when the weather is fine andwindy. Instead, it diminishes or disappears when the rain falls and plates pollens on the ground.But during storms, turbulence can stir up pollen and increase the risk of allergy.At the start of precipitation, pollen can burst and release into the air very small allergens.
  Atmospheric conditions have an influence on present pollen quantities in air:
  - Heat and humidity augment it,
  - Dryness or cold diminish it.
• More cross-reactivity are possible between pollens of trees of different species or with the fruits of these trees: for example between the pollens of birch, hazel, alder, Hornbeam and nuts, hazelnuts.
• the allergic persons are sensitized by the pollens of trees in spring and still are affected when arrive the pollens of grasses, in summer, then the pollens of Asteraceae (wormwoods, Ambrosia or ragweed, plantain) and from Urticaceae (nettles and especially Parietaria), in autumn.
  

Period of
blossoming:

Risk

Note that the pine pollen, abundant and very visible (yellow colour), does not have allergenic power. Maples, except perhaps the negundo maple (more present in Canada than in Europe), are not allergenic.
In Canada, allergies can manifest themselves with evergreens, as the red juniper, the common juniper of mountain, the sequoia, the bald cypress and the Western arborvitae.
Pollinose also exists with exotic trees: cashew (Anacardium occidentale, Cashew), coconut tree (Cokes nucifera), papaya tree (Carica papaya), palm (Phoenix canariensis). They affect the workers working in plantations. The allergic in the pollens of these trees can react to the fruits of these trees (coconut, papaya, cashew nut).

The symptoms of allergy are:
- rhinitis: blocked nose, sneezes, mucus production, itching and, "hay fever",
- conjunctivitis: red eyes which sting,
- crises of asthma: reduction of breath, bronchial hiss, often nocturnal persistent cough.

Which measure to take to limit inconveniences?
in a preventive or remedial way, it is possible to take adapted medicine. It is preferable to avoid zones and periods at risk by staying at home, by closing the windows of the car when they circulate, one avoiding efforts in zones of risk (which would demand a strong breathing and aeration of lungs), by taking into account alerts published by surveillance organisms.
If you are sensitive to allergies, avoid planting allergenic species in your garden (as the cypress and arborvitaes).
If you must maintain your garden, in spite of the risk of allergies, wash you hands, face, and even nose (by inspiring some water before blowing nose), as soon as you return home.
Avoid drying your linen outside, because the pollen can settle there and disturb you even though you did not go out.
Surveillance aerobiological consists in taking a census of presence and of density of pollens in air that can have an impact on allergic risk for the population.. It is led by an European network, the European Aerobiology Network:
You will find links towards other European national networks on their site:
www.polleninfo.org
Information pollens Switzerland:
http://www.bulletin.ch/pollen_fr.html
In France, the National Network of Surveillance Aerobiological (R.N.S.A). was created in 1996 to follow jobs accomplished since 1985 by the Laboratoire d'Aérobiologie de l' Institut Pasteur in Paris.
http://www.pollens.fr/accueil.php