Review of 'Normal background levels of air and surface mould reserve in UK residential building stock'

Dear Zuraifah,

Many thanks for reviewing our paper and apologies for our delayed response.

Please see below our responses to your individual comments:

• Line 2: Problem of mould in the UK – is that associated with indoor environment in general or only with water intrusion, damped environment? It is best to state a prevalence rate and references to support this statement.

Problem of mould in the UK, as in many other locations, have been found linked with escape of water, climatic conditions and hazards (e.g. flooding, driven rain and the like), poor management of indoor conditions including insufficient ventilation and fuel poverty. We have outlined this very briefly and added references.

• Line 6: Is ‘acceptable mould level’ refers to only airborne fungi? Or does it give weightage to common clinically important fungi which can brings effects on health or mycotoxin producers? Is there a way to address this from previous study or is it still a research gap?

Acceptable mould level refers in most guidance to airborne fungi as these documents are more focussed on the health implications of mould and the amount of airborne fungal agents is considered as the most representative of potential exposure. In our study we also address the acceptable surface mould issue, as in the common or “normal background” levels of mould.

• Line 7: Is the baseline level indoors also correlates with the mould levels outdoors, since seasonal variations may also play a role in the fluctuations of fungal spores in both outdoor and indoor air? Maybe, could add in reference(s) from previous studies?

We have now added a paragraph (third paragraph of the Introduction) on the indoor-outdoor comparison, and comments on seasonality to the discussion.

• To add in more references/background on NAHA why this method is a way forward to be used as a marker of mould contamination. It is important to know what are the advantages or disadvantages of this method as compared to other widely used methods, as a strong basis on why this method is preferred and more/equally reliable. Is there side-by-side comparison and correlation with widely used method of media plate/filter attached to air-sampler expressed in CFUs? Any reference(s)? Does the RFU correlates to culturable method or non-culturable method such as PCR? Any reference(s)? How specific is the method to capture the enzymatic activity of only moulds and not other contaminants?

More info and references on the method have been added to the end of the Introduction.

• Page 2: Line 1: A total of 140 non-water-damaged rooms with no visible mould were sampled. How did the study sites were selected? Could the sample size infer/ represent the UK residential buildings? Or, was it a survey of selected buildings? How long was the duration of the study, and do you think that environmental factors (confounding factors) such as seasonal variations, humidity level, and types of ventilation might affect the results?

The studied buildings were made available to us by volunteers. We have confidence that our selected building set is able to reflect a meaningful representation of English building stock thanks to its randomness in terms of several key parameters including location, construction type, age and occupancy types, while the preliminary nature of this examination being reserved.

We thank the reviewer for this comment especially, as it made us revisit our title, which has now been changed as the “(…) in English residential building stock (…)” from “(…) in UK residential building stock (…)”. The manuscript has also been revised to reflect this change.

For a detailed account of how humidity and ventilation might have influenced the mould readings in some of our tested rooms please see Aktas et al., 2018a. As per the seasonality, we have not differentiated our analyses based on that, however the stark similarity between our suggested thresholds for England and Denmark may suggest some level of disassociation between normal background levels and seasons. We agree that this should be looked into more deeply by future research however, and we have now added this point to our discussion section.

• To use clear terms. ‘Samples containing mould growth’……. Does it referring to growing moulds in culture media from the samples to confirm presence of culturable mould or does it refer to as ‘no visible mould growth at the sites’?

This phrase has now been replaced as “samples identified by microscopy analysis that did not contain mould growth” to avoid confusion.

• Missing link of figure (appears as error)

This has now been fixed.

• Figure 1: Missing word ‘98% of all samples from clean…...the UK and'

This has now been fixed.

• The RFU reading (x-axis) was grouped into nine scales. On what basis each cut-off of RFUs (2.7, 7.4. etc) and cut- off of 25 and 450 were chosen? Any reference(s)?

The values are as such because this is an exponential axis. This has now been clarified. The axis has been further amended to show the ranges more clearly.

• Page 7: Is this part of section 3?

Now the results and discussion have been collated under the same section to avoid confusion.