It is likely that there is a different contribution of risk factors to the various types of wheezing disorders in childhood.⁴

Family history

The link between family history, wheezing and asthma is clear. A family history of atopy or asthma (or other lower airways disease) is a risk factor for all types of wheezing, but significantly more of a risk for persistent wheezing.4 Maternal atopy and asthma in particular increase the risk of persistent wheezing.⁵

Family studies have demonstrated the genetic contribution to atopy, persistent wheezing and asthma.⁴ The familial contribution to transient early wheezing is likely to take the form of congenitally small airways.⁴

All types of wheezing are significantly more frequent in boys than girls⁴ and boys tend to have lower airway function than girls during infancy and childhood.¹⁵

Environmental tobacco smoke

Maternal smoking in pregnancy is a risk factor for all types of wheezing but not for asthma itself. It increases the risk of transient early wheezing and impaired lung function in infancy, but not in later childhood.¹⁴ It appears to affect lung development, resulting in reduced lung capacity and smaller airways, as well as prematurity and low birth weight.⁴

While smoking does not increase the likelihood of later asthma,⁷ exposure to maternal smoking in utero has been associated with long-term deficits in lung function that, together with the lung impairment produced by asthma, may increase the risk of chronic respiratory diseases later in life.¹⁶

As well as improving overall health, evidence suggests that reducing maternal smoking would decrease wheezing illnesses in young children and reduce the long-term effects of tobacco smoke on children with asthma.¹⁶

Allergic sensitisation

Allergic responses such as allergic rhinitis and eczema are associated with persistent wheezing but not transient wheezing.⁴ There is also a direct relationship between risk of persistent wheezing and serum IgE level during the first year of life, just as there is a link between asthma and IgE level in older children and adults.⁵

Despite the strong association between atopy, persistent wheezing and asthma, the role of sensitisation in the development of asthma is far from clear. There have been conflicting results from studies examining the effects of common allergens such as house dust mite and pets on the development of asthma.¹⁷

A growing body of research indicates that the prevalence of asthma is independent of allergen exposure in early life.¹⁷ Interventions to reduce exposure to allergens (such as house dust mite) can reduce wheezing in babies but do not appear to alter the development of wheezing or asthma in later childhood.¹⁸

Allergen sensitisation may not be a direct risk factor for persistent wheezing and asthma. There may be other, as yet unknown, factors responsible for both allergen sensitisation and the onset of persistent wheezing and asthma.¹⁸

Infections and the hygiene hypothesis

Environmental conditions that increase the rate of bacterial and viral infections, such as larger family size and attending a day care centre, have been found to both cause and protect against wheezing and asthma.

• Both are risk factors for transient wheezing and early onset persistent wheezing, probably related to the effect of viral infections in smaller airways.⁴
• Conversely, these same factors are protective against late onset wheezing, possibly because early infections may down-regulate the immune system against allergic responses to environmental antigens (see below).⁴

Children with frequent simple colds and other common childhood infections in infancy (without associated wheeze) are less likely to develop persistent wheezing in later childhood.¹⁹

The proposed protective effect of childhood infections is called the ‘hygiene hypothesis’. Infections are said to promote normal maturation away from the foetal type 2 immune response that increases susceptibility to allergic disease, towards the adult type 1 response which is more effective in eliminating viruses and other infections.²⁰

It is still not clear exactly which viruses protect against and which cause wheezing and asthma, and how to reproduce the protective effects of some infections while reducing the burden of other more serious infections.²⁰

Content Updated March 2005