14TH HARIDAS LECTURE: AN EVALUATION OF INDOOR ALLERGENS IN SINGAPORE

The 14th Haridas memorial lecture was held on the 5th of February this year at the Concorde Hotel. This event was organised by the Singapore Paediatric Society and the Singapore Medical Association to commemorate the late Dr Gopal Haridas who was a pioneer paediatrician in Singapore. This year, the memorial lecture was awarded to Associate Professor Lee Bee Wah of the Department of Paediatrics, National University of Singapore for her paper on “An Evaluation of Indoor Allergens and the Efficacy of Allergen Control Modalities in Singapore.” Dr Lim Teck Beng, 1st Vice-President of the Singapore Medical Association presented the award.

In her clinical study, Prof Lee assessed 96 asthmatic children and 39 atopic adults for allergen sensitisation by skin prick tests. 38 nonatopic adults acted as controls. She found more than 87% of the asthmatic children to be sensitized to both the house dust mite as well as the storage mite allergen indicating that in Singapore, storage mite is also an important allergen. In addition, a proportion of children was also sensitized to cockroach (53%), dog (29%) and cat (20%) allergens. 

956 dust samples from various localities were collected viz residential homes, schools, childcare centres, a university and a hospital. These were assessed for levels of house dust mite, cat and dog danders, cockroach allergen and storage mite allergen using specific enzyme immunoassay.

Homes were found to have significantly higher allergen levels compared to other locations. However, higher mean cockroach allergens were found in schools. Within the home, house dust mite allergen was the most prevalent and was found mainly in mattresses and carpets. Storage mite allergen was predictably concentrated in the storeroom and kitchen. Cat and dog allergens were well-distributed and not confined to homes with pets. Highest levels were found in soft furnishings. This phenomenon of the presence of contaminating allergens in areas without pets has been well-documented in other studies. There was also an absence of significant seasonal variation in mite levels in homes over a 1 year study period, consistent with our tropical climate.

The effect of various allergen control measures were evaluated by chemical  acaricide applications, allergen barrier bed covers and high efficacy particulate air (HEPA) filters.

Serial measurements of mite allergen levels over a four month period were carried out following implementation of the control modalities in residential homes. Untreated homes acted as controls.

Chemical acaricide application (D’Allergen) and use of allergen bed barrier covers (Allergy control cover) were found to significantly decrease allergen levels for up to 2 months post-implementation. Hence the need for repeated applications and regular washing of these covers at 2-3 month intervals. In contrast, HEPA filters did not have any significant effect on decreasing allergen levels. This is probably due to the relatively large particle size of dust mite allergen and their tendency not to be airborne. 

However, these commercially produced mite allergen control modalities are to be used as supplementary measures as stringent simple cleaning practices per se were effective in decreasing dust mite levels.

In conclusion, house dust mites form the most important indoor allergen with the storage mite constituting one of the major allergic components of dust in Singapore. Compared to public places, the home constitutes the major reservoir of indoor allergens, especially in mattresses and carpets. Stringent simple cleaning practices supplemented with the selective use of allergen control modalities - namely chemical acaricide application and allergy bed barrier covers are effective in decreasing dust mite levels.