Worldwide distribution
Dogs have a global distribution and an estimated total population size of 700 million (14). Dogs have evolved a close relationship with humans and can be found as owned pets in households or as free-roaming strays in urban and rural environments (14). Concentrations of dog allergens are highest in indoor environments where these animals are kept, but can also be detected in other indoor or public places where dogs have never been kept because of passive transfer (3, 4).
Living environment
Dogs release allergens through secretions and on tiny particles of skin scales (dander) between 2–5 microns in size (1 micron = 1/25,000 inch) which are commonly found in household dust (4, 15, 16). These particles easily become airborne under normal ventilation so can disperse effectively, and their tiny size enables them to enter small bronchioles and reach lower airways to trigger respiratory symptoms in sensitized individuals (3, 4, 16, 17). Dog hair and dander extracts are complex mixtures of components with more than 28 antigens, any of which can bind to IgE antibodies with varying frequency and intensity in dog-sensitive patients (18).
All dogs produce allergens, however, there are differences between breeds and between individuals within breeds (17, 19-21). Certain breeds are more susceptible to eczema and oily seborrhea, while older dogs have drier skin and produce more dander than younger dogs (22). Seasonal variation can also affect the levels of dog IgE antibodies (22).
All homes with dogs have high levels of dog allergens (23, 24). In private homes, dog allergens have been measured at 1 to >10 micrograms per gram of dust (4, 16), and airborne Can f 1 can range from 0.3 to 99 nanograms per cubic meter of air (16). Levels in homes without dogs maybe 10–100 times lower, but are still detectable and can affect sensitized individuals (4, 16, 23, 24). The threshold level for sensitization to Can f 1 is >2 micrograms per gram, while the threshold level associated with asthma symptoms in sensitized individuals is >10 micrograms per gram (25).
In both homes with and without pets, the highest levels of pet allergens in dust reservoirs and air are found in the living room and bedroom (16, 23, 24). Carpets and upholstery are major reservoirs for dog allergens, however, detectable levels are also present on bare floors and smooth surfaces such as walls and furniture (16, 26).
School
Apart from the home, school is the most important indoor environment for children (25, 27). Many studies have detected dog allergens in schools at levels exceeding those needed to induce sensitization (27-30). However, levels of dog allergens can vary extensively both within and between schools, depending on factors including the presence of open shelving with settled dust, the extent of carpeted or upholstered areas, and the number of pet owners visiting the school (4, 25, 27, 29).
Classroom fittings, particularly chairs, can act as both direct and indirect sources of different particles and compounds, and the concentration of animal dander allergens in school dust frequently exceeds the amount measured in homes with no furred pets (25, 27, 28, 30). This means that children with asthma and other allergic diseases can be exposed to dog allergens at school even if they have no dogs at home (29, 30). The main source of animal allergens brought into the school setting is the clothing of pet owners (31). It is possible to reduce the levels of animal allergens if students change into school uniforms which are stored and washed at school (31). A Swedish national position paper on asthma and allergies at school recommended that furred animals including dogs should not be allowed on school premises at any time, in order to decrease the allergen levels in favor of pet allergic pupils (32).
Other topics
Significant exposure to domestic allergens can occur outside homes (33). The particles that carry dog allergens are sticky (3), and passive transfer on clothing or human hair can easily transport them to places such as schools, offices, hospitals, automobiles, public transport, and other places where dogs are not usually present (25, 29, 34-36). In these areas, furnishings, textiles such as curtains, upholstery and dust can act as significant reservoirs of allergens and impact indoor air quality (27, 33, 35, 36) in levels which may be higher than those in homes without a dog (34). While levels of allergens may be relatively low in environments outside the home, this exposure could be important for sensitized individuals who do not have a pet at home (29, 35, 36).
For example, upholstered chairs in hospitals constitute a significant reservoir of dog allergen particles, which if inhaled by patients attending appointments could exacerbate asthma (33). Additionally, a public transport study in Helsinki found just over half (53%) of passengers with allergy or asthma had been inconvenienced by symptoms during travel, even though only 0.13% of passengers travelled with a pet (36).