21 March 2012
By Dr Tom Jeavons and Professor Michael Abramson
We know that Australia is a country of droughts and flooding rains, but recently it has seen more rain than usual. In the past five years there has been record rainfall and flooding in many towns and cities across eastern Australia. The floods themselves are dangerous, but there are also health hazards associated with the indoor dampness that follows, and more specifically the excessive mould this encourages.
There were extreme cases of dampness and high levels of indoor mould growth following hurricane Katrina and flooding of homes in New Orleans in 2005. Concern for the health of workers involved in the clean-up led the United States Centres for Disease Control to advise that “excessive exposure to mould-contaminated materials can cause health effects in susceptible persons regardless of the type of mould or extent of contamination”.
Two major reports to the United States Institute of Medicine (IOM 2000; 2004) concluded that there was sufficient evidence of an association between mould and upper respiratory tract symptoms, wheeze, cough, and asthma symptoms in sensitised asthmatic individuals. Although there is a strong body of evidence linking indoor dampness to asthma attacks, the exact role of moulds has been difficult to determine, because it has not been easy to disentangle mould exposures from the many other exposures that occur in damp indoor environments.
Persistent dampness causes building materials to decompose and release volatile organic compounds (VOCs) - phthalates, for example, are released from wet vinyl floorings. Mould exposure is complex because residents are exposed to both living and dead spores and to mould fragments and also many mould products - allergens, cell wall chemicals, toxins, and microbial VOCs.
The risks to respiratory health depend on the levels of mould exposure. It is a well-known principle of toxicology that ‘the dose makes the poison’. In the case of damp houses, the ‘dose’ is the concentration of airborne spores and fragments of moulds inhaled through personal exposure.
Currently there is no government or industry standard for acceptable levels of indoor airborne mould (WHO 2009). Measuring them accurately is difficult. Also, the threshold numbers that provoke attacks of asthma are likely to vary with the precise moulds present, and will also depend on the varying susceptibilities of the exposed individuals.
For asthma symptoms, the health risks depend on allergic sensitisation. Our team at the Monash Department of Epidemiology & Preventive Medicine conducted a study of environmental causes of asthma in 1996. We found that 38 per cent of our sample of allergic adults had a mould allergy. Most of these were also sensitised to other allergens, predominantly house dust mite. Poly-(multiple) sensitisation makes the contribution of mould allergy to asthma severity difficult to assess. However our study suggested that mould-allergic adults had twice the risk of asthma attacks and wheeze, and were twice as likely to use asthma medication, when compared to other allergic adults.
Clinical studies also suggest that mould allergy is associated with more severe asthma, but more research is needed on mould exposure and associated health effects for us to have confidence that a causal relationship exists.
Even though the effects of moulds on health are not fully understood, the World Health Organization recommends that dampness and mould-related problems be remediated because “they increase the risk of hazardous exposure to microbes and chemicals”. (Remediation usually involves removal of mould-damaged building materials and furnishings, and the use of fungicides.)
A recent Cochrane Review (2011) found that remediation reduces asthma-related symptoms, respiratory infections and also decreases the use of asthma medication. In our later pilot study, a randomised controlled trial of 13 mould-allergic adults showed that dehumidification and cleaning of bedroom surfaces with diluted bleach reduced airborne mould levels and was associated with a significant improvement in peak flow measurements.
Why does this matter? Because, although there has been ongoing debate about whether the level of flooding we have recently experienced is due to global warming superimposed on natural Australian climate cycles, in the near future we are likely to see an increase in flooding and more exposure to moulds. The Climate Commission (2011) believes that rising ocean temperatures, changes in rainfall patterns, and extreme weather events will lead to increased relative humidity and more exposure to airborne allergens such as pollens and moulds.
The climate paper just released by CSIRO and the Australian Bureau of Meteorology provides further evidence of a continuing warming trend, and leads us to think that the risk of mould-related respiratory problems will only increase in the future.
Dr Tom Jeavons is a Lecturer in the Department of Microbiology and Professor Michael Abramson is the Deputy Head of the Department of Epidemiology and Preventive Medicine at Monash University.
A version of this article appeared on The Conversation.