Infants who are at risk of asthma are also likely to be at risk of atopy. It is not certain that measures to reduce house dust mite (HDM) levels in bedding reduce the risk of asthma, but they are likely to reduce the risk of sensitisation and subsequent development of atopic disease.

The amount of allergen necessary to sensitise genetically at risk children is unclear.¹¹ However, both prospective and cross-sectional studies have shown a dose-response relationship between the level of exposure to HDM allergens and allergic sensitisation.^{4, 11, 22, 27-33}

Mattresses and under bedding, quilts and pillows have been found to be significant reservoirs of HDM allergen.^{9, 10, 34} Many studies have been conducted to determine the efficacy of encasing bedding items to reduce the levels of HDM allergens. Recent studies have generally found a positive effect as well as compliance with the use of the encasings:

• the Prevention and Incidence of Asthma and Mite Allergy (PIAMA) study found that applying allergen-impermeable mattress encasings to infants’ and parents’ beds before birth, reduced HDM allergen levels about half³⁵
• a randomised controlled trial into the effect of the use of mattress and pillow encasings coated with semipermeable polyurethane found a significant reduction in HDM allergen concentrations in the intervention group and high compliance with the intervention³⁶
• the Manchester Asthma and Allergy Study found that the use of allergen-impermeable mattress encasings (combined with uncarpeted floors) was effective in substantially reducing levels of HDM allergen in the long term and was acceptable to families³⁷
• a review of controlled trials found that installing allergen proof encasings and frequent washing of bedding reduced HDM exposure by 10-fold or more³⁸
• a randomised controlled trial in which mattresses, pillows, and doonas were encased and bedding was washed in hot water found that HDM allergen was reduced to one-third from baseline, which was significantly lower than the control group, even at 24 months.³⁹

While these findings suggest a role for bedding covers in the reduction of HDM allergen levels, it should be noted that two recent randomised controlled trials have questioned their efficacy in this role.⁴⁰

As well, Cochrane review of HDM control measures in asthma management found that the available evidence on chemical and physical approaches did not provide a secure basis for advice and policy for asthma prevention.⁴¹

Reducing exposure to HDM allergens may reduce wheezing in babies but does not appear to alter the development of wheezing or asthma in later childhood.¹ It appears likely that other, as yet unknown, factors may be responsible for both allergen sensitisation and the onset of persistent wheezing and the onset of asthma.^{1, 11, 35-39, 42, 43 <LE II>} in more frequent symptoms.^{<LE III-2>}

Washing bedding

The effects of various laundry procedures on house dust mites and their allergens have been examined. Studies have investigated various washing temperatures, the use of laundry products, and the effectiveness of dry cleaning in removing mites and allergens.

Washing in cold water, with or without laundry powder, and with or without bleach has little effect on mites but does reduce allergen concentrations.⁴⁴, 45 Washing at 25°C extracts almost all allergens within 5 minutes.⁴⁶ However, washing at a minimum temperature of 55°C exterminates mites^{44, 47} and results in a lower residual amount of HDM allergens.^{8, 46}

Detergents have been found to extract more HDM allergen that either soap or water alone.⁴⁶ A range of commercial detergents have been found to have a similar effect on levels of mites and allergens, with enzyme-containing formulations having no greater effect.⁴⁶ With most laundry products, washing at >60°C is required to kill mites but products containing essential oils, such as tea tree or eucalyptus oil, are formulated to kill dust mites and may be effective in cold water.⁴⁸

The use of acaricides, such as benzyl benzoate, and other substances (eg tannic acid) has not been shown to reduce HDM allergen levels.^{39, 49-51}

Dry cleaning has not been found to reduce HDM allergen concentration in bedding, although most, if not all, mites were killed.⁴⁴

Studies have also shown that HDM allergens accumulate rapidly in soft toys.⁵² For this reason soft toys that can be washed in hot water should be selected³⁸ and washed regularly.

Washing sheets, pillow cases, cotton blankets and soft toys weekly in hot water (55°C) will kill mites and wash away the allergen they produce.^{<LE II>} 

Sheepskins

Sheepskins are a common form of under bedding in Australia and New Zealand. The sheepskin comprises the animal hide with the natural wool fibres attached. Infants are typically placed directly on the sheepskin.

Because of dampness, warmth and frequent use, sheepskin under bedding can rapidly accumulate high populations of house dust mites.^{5, 53, 54} An association has also been found between the use of sheepskins and increased HDM sensitisation.²²

Due to their rapid accumulation of high populations of HDM, the use of sheepskins in the beds of infants at high risk of atopy is not recommended.^{22, 42, 55 <LE II>}

Content Updated March 2005