Mould Testing and Benchmarking: a public report

Mould Testing and Benchmarking: a public report

The understanding of indoor mould is complex and sometimes contentious. Reputable organisations including the World Health Organisation (WHO), the US based Institute of Medicine (IoM) and the National Health Service (NHS) all affirm that high levels of mould in buildings are unhealthy for occupants, and can lead to respiratory and other health problems. Furthermore, moulds can clearly cause fabric decay as well as surface degradation and staining. The environmental conditions and building context within which such moulds develop and affect the building fabric are relatively well understood. However, moulds which might affect human health are not so well understood, partly because the connections between moulds and health are highly complex, but also because we do not currently have a standardised or robust method of measuring indoor mould, or benchmarks for what is a “normal” or “acceptable” level in houses. The research outlined in this report is an attempt to address this problem of measurement, and provide a way forward for research in this area.

Why is it difficult to measure mould levels in buildings? The main reason is that most mould is not visible, but is airborne. So in order to understand the condition of a house, we have to measure the levels and perhaps also the types of mould in the indoor air (as well as sampling any visible mould). But how do we measure mould in the air? And what type or part of mould should we be seeking to measure? How do these also relate to mould on surfaces? It is because these questions have not been adequately answered in independent rigorous research that it is currently impossible to say what an acceptable level of mould in a building is, and at what point mould concentrations exceed this level.

This project involved an extensive literature search of mould testing techniques and methodologies, a programme to test certain methodologies, an analysis of the effect of environmental and lifestyle factors on the tested mould levels, and finally an attempt to benchmark mould levels under the chosen test protocol. The conclusions to the project are as follows:

  1. There are currently no established techniques for mould testing which would allow proper comparison of test results reported by different studies, and enable the benchmarking of mould levels in buildings. The establishment of an accepted methodology and protocol for testing mould levels is essential if meaningful research into the effects of mould on human health, and the relationship between building condition and mould levels, is to be possible.
  2. It is unclear which mould species or parts of moulds (such as spores, cell fragments, mycotoxins, beta glucans) affect human health and therefore should be measured. It is estimated that there are over a million species of mould, and indoor air will normally contain thousands of mould spores in each cubic metre, but there are no clear definitions or methods of testing for most of them. Furthermore it is unclear how the human body responds to different types and parts or combinations of moulds, as this is also dependent not only on exposure but on other conditions of the individuals concerned (such as age, health, genetic make-up, lifestyle, occupation). In the context of this considerable uncertainty and complexity, the method that we used in this pilot research for assessing mould in buildings (following guidance from New York City Department of Health, among others) is to measure the general level of all types and parts of moulds, rather than individual species or parts1.
  3. This general level testing was achieved by a measurement technique based on the quantification of the activity of N-acetylhexosaminidase (NAHA), an enzyme which has been found to be a reliable marker of fungal cell biomass. This testing was carried out using an active (aggressive) sampling method. We also field-tested passive (non-aggressive) and active (aggressive) sampling techniques using a culture- based and a particle counting method. On the basis of these results as well as an analysis of the literature, it appears that passive testing of the air in buildings is unable to capture visible mould in the tested room, and can greatly underestimate the concentration of moulds within a given indoor environment. It is one of the conclusions of this project that active sampling methods should therefore be used in mould testing. Active methods involve controlled air-blowing in order to disturb mould fragments in the room being tested and thereby provide greater consistency and reproducibility to air sampling, while at the same time replicating more realistically some level of human activity in a room, and making obtained readings more representative of the actual exposure levels.
    1. The combined methodology of active air sampling and particle counting as well as surface sampling from visually clean and dirty/dusty surfaces, together appear to be a reliable basis for mould testing in a room. The air and surface readings obtained from indoor environments with no visible mould at all were then analysed using a benchmarking methodology used by the Danish Building Institute. Importantly, benchmark values suggested here are only valid using the exact same testing protocol, and were found quite similar to benchmarks previously established (and currently in use) in Denmark by using the same methods of testing and analysis.
    2. However, other active testing methods and types of sampling may also allow benchmarking. From a research perspective, it is important that the majority of research work strictly follows the same protocols, whatever these are, so that results are comparable and a significant body of data can be built up.
    3. The results show a clear difference between mould concentrations obtained from homes with visible mould and those without. These benchmarks could be used to identify the degree of the mould infestation, and, possibly, with further research, hidden (i.e. non-visible) mould infestation.
    4. Through survey sheets and questionnaires, developed here and used in conjunction with the testing methodology, this research also looked into how building condition, context (e.g. age, location, construction type, insulation, function and level of furnishing) were related to mould levels, particularly where mould levels are high. This could lead to positive measures to reduce mould levels through changes to the building and behaviour. However, in this short study we found only very limited correlations, which indicates the need for a greater and longer testing programme.

    It should be stated again that this project is very much a first step in our attempts to test and benchmark mould levels in buildings, and, by doing so, to bring greater clarity to the relationship between buildings, mould and people. It is important that we build on this first project with further research both to widen and to challenge these early findings, so that moisture safety in buildings can be taken forward in a logical and proven manner.


Yasemin D. Aktas

Hector Altamirano

Ioanna Ioannou

Neil May

Dina D’Ayala