Asthmatics breathe easier in mould-free homes

Thursday September 6 2007

Cleaning mould from asthma sufferers’ homes could ease their symptoms, The BBC reported. The news report was based on a study that found that people with asthma who lived in mould-free homes used their inhalers less and symptoms like sneezing lessened.

The study was an unblinded randomised controlled trial in which households where asthmatic people lived were either cleaned of mould or left mouldy for a 12 month period.

Analysis of the results found that there were differences between the groups in how they ‘perceived’ their symptoms. However, the lack of response in objective asthma measurements suggests that this intervention will need further evaluation if it is to become common practice. The fact that people felt better following an intensive “spring clean” of their houses is not surprising but could be attributed to a placebo effect.

While we wait for further research that could show a real response according to the objective asthma measurements, there is no harm in removing mould from the home.

Where did the story come from?

Doctors Michael Burr and colleagues form the Cardiff University department of epidemiology, statistics and public health conducted the research. Funding was received from Asthma UK, the Medical Research Council and the Welsh Office of Research and Development.

The study was published in the peer-reviewed medical journal: Thorax.

What kind of scientific study was this?

This was an unblinded, randomised controlled trial. People on the asthma registers held by GPs or who had participated in surveys on housing and health were invited into the study by letter. This resulted in 164 households (with 232 occupants) taking part in the study and being randomly separated into either experimental or control groups.

The experimental group had 81 houses which received a thorough clean with detergents and a fungicide designed to kill all visible and invisible surface mould.

At 6 and 12 months into the study, participants were given a questionnaire asking about their asthma symptoms and their use of asthma inhalers. They were also tested for peak flow rate (an objective measurement of narrowing of the airways).

During the experiment, the control group did not have their mould removed. However, it was removed after 12 months, so that all participants eventually had their houses cleaned.

Of those houses that took part, data was not available for 13 of the 81 cleaned houses or for 20 of the 83 houses used as a control group.

What were the results of the study?

The responses to the questionnaire showed significant differences between the groups after the first six-month period had passed. This was in the responses to questions about wheezing that was sufficient to affect breathing and in their perceived improvement of breathing.

The variability in peak flow rate declined in both groups and there was no significant difference between groups.

What interpretations did the researchers draw from these results?

The researchers concluded that “although there was no objective evidence of benefit, symptoms of asthma and rhinitis (itchy runny nose) improved". They claimed that it was unlikely that this was due to a placebo effect.

What does the NHS Knowledge Service make of this study?

The researchers state that previous studies which looked for associations between mould and asthma may have been influenced by confounding factors such as socio-economic status, lifestyle, housing and other issues. The randomisation in this study has removed many of these previous problems which occur when selecting houses to be part of intervention and control groups. 

There are some limitations to this study which the researchers acknowledge.

  • A blinded study, where the occupants of the houses were unaware of whether their house had been cleaned or not would have been impossible. This means that the objective measures of asthma or rhinitis (the test of peak flow rate) become more important so any placebo effect can be effectively ruled out. The placebo effect is a well known bias that can arise in studies that are unblinded. In this study people who were aware that their house had been cleaned may have subconsciously reported favourable improvements in their symptoms, irrespective of any actual change in asthma severity.
  • The follow up rate of households (i.e. number of households available for measurement throughout the study) was not high. At six months, data was available from only about 60% of households.

As there was no observed improvement in asthma as measured by the objective peak flow rate, it would be premature to recommend that a process of household mould eradication should be part of routine practice for asthmatics.

However, there is no harm in removing mould from the home, and households with people with asthma might find they get some benefit, even if it is just a placebo.

Sir Muir Gray adds...

Absence of proof of effectiveness is not proof of ineffectiveness; parents who have been trying to reduce the level of allergens in the home should not stop on account of this single paper.

Analysis by Bazian
Edited by NHS Choices