The most common allergens include:

House dust mites

House dust mites, which are part of the spider family, measure between 0.2-0.4mm and are prevalent in all households, where they tend to be more numerous in bedding, upholstery and carpets. High levels of exposure to house dust mite allergens can occur throughout the day in a range of indoor environments both inside and outside the home¹ 

House dust mites are one of the major causes of allergic rhinitis with symptoms such as congestion, sneezing, a dry cough or bronchitis which can lead to asthma.

Due to year round allergen exposure, symptoms due to house dust mite allergies are perennial.

Pollen

Grass pollens are the most common allergenic pollens implicated in respiratory allergies in Australia²

Trees and weeds can also be significant sources of allergenic pollens. Pollens are suspended in the air and can be carried by the wind over large distances (e.g. from pasture lands to urban environments).  In addition to the action of the wind, humid weather conditions (particularly thunderstorms) can exacerbate pollen exposure by releasing microscopic pollen fragments (<1 µm), which can penetrate easily into the lower airways to cause asthma symptoms³

Symptoms due to pollen allergies are seasonal, occurring when different allergenic plants undergo flowering, and the timing of symptoms can help to indicate which pollens are important allergy triggers.

Grass pollen

The Poaceae (grass) family includes more than 12,000 different species. Grasses are found in a wide variety of environments including pasture lands, roadsides, gardens and lawns. Even a tiny concentration of grass pollen is enough to trigger an allergic reaction.

Ryegrass and related temperate grasses (such as cocksfoot, orchard grass, sweet vernal grass, meadow grass and timothy) are major pasture grasses and are important allergic triggers during spring and early summer⁴ 

Tree pollen

Tree pollens which can trigger allergies include:

• the Betulaceae family (birch)
• the Fagaceum family (beech, oak)
• the Oleacceae family (ash, olive tree)
• the Cupressaceae family (cypress, juniper, cedar)

Their seasonal pollination period is spread over several months and varies from one species to another.

Weed pollen

Species from the herbaceous family have soft and supple stems. Their pollen can remain suspended in the atmosphere for a long period of time and can be carried over large distances.

Herbaceous weeds that can trigger an allergic reaction include: plantain, wall pellitory and ragweed.

Pets

Allergies to cats represent two thirds of allergies to pets. Contrary to common belief, what triggers an allergic response isn’t pet hair but a substance (Fel d 1) found on the pet’s fur.

This protein (or allergen) is produced by the skin of felines and is also present in their saliva, urine, tears and dander.

The allergen can be found throughout homes, in bedding, upholstery, rugs and can also be suspended in the air. Cat hair can also be transported into environments outside the home by clothes and shoes.

Mould

Mould is composed of microscopic fungi that are present in the environment. Fungi can be found both indoors in damp areas (bathroom, kitchen, etc.) and outdoors (fallen leaves, compost, grasses, etc.). The spores released by the fungi can be carried by wind and dew. Mould can also develop on foods (bread, cheese, fruit and vegetables).

The microscopic size of fungi spores (3-10 µm), enables them to easily penetrate the respiratory tract.

The most important mould for respiratory allergies in Australia is Alternaria alternata, with high prevalence both indoors and outdoors. Outdoor exposures can show a seasonal pattern, with lowest exposures in winter⁵ 

1. Tovey et al Most Personal Exposure to House Dust Mite Aeroallergen Occurs during the Day PLoS ONE 2013; 8(7): e69900

2. ASCIA Information for Health Professionals ‘Aeroallergen Immunotherapy Selection Guide’ 2019. Access here: https://www.allergy.org.au/images/pcc/Aeroallergen_Selection_Guide_2019.pdf

3. Buters et al JACI 2015; 136:87-95; Hew et al Allergy. 2020 Sep;75(9):2369-2372. doi: 10.1111/all.14319. Epub 2020 May 4

4. 5. ASCIA Information for Health Professionals ‘Aeroallergen Immunotherapy Selection Guide’ 2019. Access here: https://www.allergy.org.au/images/pcc/Aeroallergen_Selection_Guide_2019.pdf