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Mould Hazards in Marijuana Grow Operations

Caoimhín P. Connell
Forensic Industrial Hygienist

On September 13, 2012 the Division of Environmental and Occupational Health Sciences, National Jewish Health (NJH) located in Denver, Colorado, announced the release of an unpublished report prepared by staff members of National Jewish Health. Due to the numerous foundational errors contained in the National Jewish report, an exhaustive critical review of the report would be cumbersome in HTML format. Therefore, the following web discussion is a very brief summary of the critical review developed regarding the National Jewish report. The full critical review, however, is available for download by clicking HERE

The National Jewish report is titled “Health Effects Associated with Indoor Marijuana Grow Operations” and we have provided a link to the report in the text below. The report lacks credibility, and failed to demonstrate an hazard to first responders.

According to a press release, 1 the report was “designed to provide advice about precautions and protective equipment law enforcement agents should wear when investigating indoor marijuana grow operations.” However authors of the report then recommended personal protection equipment that could harm law enforcement personnel who followed the recommendations.

Legitimate researchers involved in law enforcement and occupational health and safety for first responders know that the primary hazards facing first responders who enter marijuana grow operations are:

Slips/trips and falls
Violent criminals
Structural hazards
Electrical hazards
Thermal burns
Booby traps
Residual chemical hazards

However, the report concluded that hazardous levels of moulds were found at indoor marijuana grow operations and that these moulds presented the primary hazard facing first responders entering grow operations.

According to the authors of the report:

A number of reports have suggested that the principal concern in indoor marijuana grow operations is the presence of excessive mold spore levels due to the elevated temperatures, humidity, and organic material in these operations. Our study has confirmed this concern.

In fact, no other studies have ever suggested that the principal concern in indoor marijuana grow operations is the presence of excessive mould spore levels; and the NJH report did not find particularly elevated spore concentrations. Since the authors of the National Jewish report have not included a single referenced document, or a single scholarly citation, no one will be able to identify these mysterious "other studies."

According to the press release:

Police and other first responders may be exposed during busts of illegal marijuana- growing operations to dangerous levels of mould that could lead to potentially deadly respiratory diseases, researchers said Monday.

Within hours following the release of the report, undue alarm spread through the law enforcement community; first responders across the country and as far away as England were contacting the offices of this reviewer (Connell) for information regarding the reported hazards.

We have reviewed the report issued by National Jewish Health (NJH) and upon reviewing the report, we have concluded the following:

• The conclusions by the authors are not supported by their data.
• The authors used improper mathematical manipulations in the presentation of their data.
• The authors make recommendations that can harm first responders (such as recommending that officers with pulmonary problems wear an N-95 or P-100 disposable respirator).
• The authors failed to employ scientifically valid sampling protocols in their assessments of indoor moulds.
• The authors employed toxicological precepts that are untested and unscientific.
• The concentrations of the moulds reported in the marijuana grows were not valid.
• The concentrations of the moulds reported in the marijuana grows were not particularly elevated.
• The concentrations of the moulds reported in the marijuana grows were significantly lower than mould concentrations reported from other agricultural operations, and which are generally regarded as safe.
• The report contained many technical errors and false assumptions that are not supported by accepted science.
• The report does not contain any information, or sampling data, within any known confidence that indicates moulds present an otherwise unrecognized health hazard to first responders.

A copy of the full FACTs review can be downloaded by clicking here.

A copy of the NJH Report can be downloaded by clicking here.

General Comments
An exhaustive critical review of the report would be difficult due to the foundational errors contained therein. Therefore, no attempt has been made to exhaustively review the NJH report. Rather, the objective of the review was to provide sufficient information to demonstrate that the report lacks credibility, and failed to demonstrate an hazard to first responders.

In general, the document is poorly written from a style and grammatical perspective and is fraught with misspelled words. For example, the authors were incapable of correctly spelling the genera “Penicillium” and “Stachybotrys” and consistently misspelled the name of the Andersen samplers that were employed. The document is full of poor grammar, unsupported propositions, and convoluted statements such as the following:

In addition, we expect the species inside the house to be similar in abundance and species to the species and abundance outside.

It would appear that the document was prepared without the benefit of any kind of internal review, peer review, proofreading, editing or consideration to fact-checking or technical vetting for scientific validity.

Throughout the document, the author(s) make many unsupported statements without any references or citations. For example:

Emergency personnel and law enforcement officers entering these facilities on a regular basis have reported upper respiratory irritation, skin rashes, and other symptoms associated with these exposures.

Many of the references are presumably not cited since there is no actual reference, and the statement is in fact contrary to published data. For example, the following unsupported quote appears in the NJH report wherein the authors have not provided any references:

A number of reports have suggested that the principal concern in indoor marijuana grow operations is the presence of excessive mold spore levels due to the elevated temperatures, humidity, and organic material in these operations.

In fact, there are no valid studies, to the knowledge of this reviewer that have suggested this conclusion. Large portions of the NJH report appear to be the same language as that which appeared in another document 2 co-authored by one of the NJH report authors3 wherein similar unfounded statements appear; for example in the following quote:

The presence of fungal growth in IMGOs has resulted in fungal contamination in many marijuana samples. McLaren et al. found that 13/14 marijuana samples were contaminated with excess amounts of Aspergillus.(12)

The cited reference (given as footnote 12) is for “McLaren, J., W. Swift, P. Dillon, and S. Allsop: Cannabis Potency and Contamination: A Review of the Literature. Addiction 103(7):1100–09 (2008).” However, if one goes to the McLaren citation, one finds that nowhere within the cited literature does the referenced statement of finding occur. That is, fact-checking reveals that McLaren et al did not report finding that 13/14 marijuana samples were contaminated with excess amounts of Aspergillus.

Throughout the present NJH document, dozens of such assertions are claimed to be shared by other authors, other studies and other reports – but nowhere in the NJH document have the authors identified those authors, or provided standard citations of any of the literature, authors, reports or studies to which they allude.

Throughout the document we also find conclusions and observations that are not supported by the NJH data; and in fact, in many cases, the NJH conclusions are actually contradicted by their own data. For example, the authors conclude:

These data indicate that the number of MGO’s with elevated spore levels appear greatest when the number of plants exceeds 50.

The actual data presented by the authors in the NJH document, demonstrates the opposite, and as the number of plants increased in the marijuana grows studied, the average spore concentrations are lower, not higher on a per plant basis. (See Figure 1, below).

Figure 1
Spore Concentrations as a Function of Plant Number

In other places, the NJH statements are in stark contradiction to themselves and to published literature and accepted facts. For example, on Page 7 of the NJH report, the following statement is made:

Since not all mold spores that are captured using the Anderson (sic) Cascade Impactor are able to grow due to viability issues, the non-viable spore levels are usually higher than the viable mold levels.

(Throughout the NJH report, the Andersen sampler is incorrectly spelled “Anderson”). In fact, the statement is simply not true and it has long been known that the sampling protocol used by the NJH authors is not capable of the representative enumeration of airborne spores, and when sufficient numbers of samples are collected in a valid manner, we see a complex relationship between “total” spore traps and the Andersen N6 samplers used by NJH. At the altitude of the NJH studies, low spore counts are biased high with the Andersen N6 sampler (vis-ŕ-vis total spore traps) and at higher spore concentrations, the “total” spore traps are favored; parity occurring at approximately 200 spores-CFUs per cubic meter of air. The graphic below depicts the results of a study of simultaneously collocated slit impactor samples compared with Andersen N6 samplers. 4

Figure 2
Comparison of Spore-traps and Andersen Samplers

The physics behind this phenomenon have been well understood for decades. However, when the NJH author(s) encountered this phenomenon it was contrary to their expectations and the author(s) concluded:

The biggest difference between the two tables are the results for MGO#14 where the viable levels of spores were much higher than the number of counted spores. The reason for this discrepancy is unknown at this time.

The reasons for the observed phenomenon are very well known to established science and several of the issues were in fact described by Andersen in 1958. 5 The statements in the NJH report exhibit a fundamental lack of understanding of the sampling equipment used in the study. Throughout their report, the NJH authors refer to the erroneous notion that “total” spore counts were enumerated or “total viable counts” when in fact, the sampling equipment used by NJH was entirely incapable of such “total” enumerations. The Andersen sampler employed and the slit impactor employed by the authors were never designed to be total spore traps, and neither are capable of representing total spores (or even total “viable” as erroneously stated by the authors). The physics upon which the two samplers work are so completely different that it is impossible to compare a slit impaction “spore trap” result to the Andersen N6 sampler without very complex mathematical standardizations – none of those were employed, and the NJH authors merely compared raw values between the samplers.

Contrary to that presented by NJH, each sampling method has only a limited inherent ability to enumerate specific types of spores. NJH erroneously refers to “total spore counts” in their report; however, the sample devices (the spore trap and the Andersen samples) each have complex collection efficiencies that are very well established. 6

Unfortunately, the authors never identified the actual type of slit spore trap they employed. Nevertheless, all such impaction spore traps (as exemplified by the Andersen cascade impactor and Air-O-CellTM sampler and others) have a specific and known “cut-size” associated with the sampler. The “cut-size” is the aerodynamic diameter, in micrometers of a theoretical spherical particle of unit density that has a 50% chance of being captured and is designated “d50.” At normal temperature and pressure, the d50 for the “total spore trap” used by NJH (if it was one of the common commercially available slit-impactor spore traps) is reported as around 2.3 µm. 7

This means that a mould spore whose diameter is approximately 2.3 µm has only a 50% chance of being captured. Now, by comparison, the Andersen sampler used by NJH has a d50 of only 0.65 µm8 which means that even on the face, it is impossible to compare Andersen samplers with the impaction spore traps since both devices have completely different capture characteristics and even under identical atmospheric conditions produce completely different results.

Importantly, the preponderance of organisms that we see in indoor air, as discussed by the NJH authors, belong to genera such as Cladosporium, Penicillium and the Aspergilli. The spore diameters for these organisms happens to be exactly within the same range as the cut-size for the devices NJH called “total” samplers. The Cladosporia (e.g. C. cladosporioides) have a diameter of 2.1 µm, the Aspergilli (e.g. A. versicolor) 2.4 µm, and members of the Penicillia (e.g. P. brevicompactum) have a diameter of 2.2 µm. 9

We explore this further in the full review, found by clicking here.

Similarly, in other places, the conclusions of the author(s) are not supported by their own data. For example the report states (on page 10):

There is strong agreement between both the viable and non-viable samples.

In fact, the data present in the report indicates there is virtually no agreement between the two data sets presented in the NJH report. When we match up pairs for the data sets for each grow operation for which the author(s) had slit impactors (erroneously referred to in the document as “non-viables”) and Andersen samples (erroneously referred to by the authors as “viable”), we see there is virtually no agreement between the two data sets. In the graphic below, we have plotted the NJH Andersen samples against the NJH slit impactor samples. Graphically “good agreement” would have been represented by a straight sloping line and a linear regression of better than 0.9 (unitless). By applying a standard linear regression, we have compared the two data sets; one as a function of the other (see below).

Figure 3
Correlation of NJH Andersen Results with Slit Impaction Results

We see that the correlation is extremely weak (0.1894); when we reversed the variables the correlation was no better (0.0884). Therefore, there is nothing in the data to support the claim “There is strong agreement between both the viable and non-viable samples.”

During the press release the authors claimed:

Study finds perilous mold in Colorado pot-growing operations10

When one reads the above referenced media account, one finds the statement:

A team working with National Jewish Health researcher Dr. John Martyny reviewed conditions in 30 marijuana- growing operations in Denver, Littleton and Larimer County and found mold levels at times 100 times higher than considered safe and in a few cases so high that their instruments could not read the levels.

There is no level of mould spores above which the exposure is considered unsafe. Therefore, at no time could NJH have “found mold levels at times 100 times higher than considered safe.”

In fact, nowhere in the NJH document have the author(s) documented, supported, or substantiated any such finding.

A false alarm has been raised to which first responders are reacting. There is a pressing need to ensure that financial resources and anxieties are reserved to actual hazards present in marijuana grow operations. A copy of the full review can be downloaded by clicking here.


1 Jason Pohl The Denver Post, September 11, 2012

2 Clandestine Indoor Marijuana Grow Operations – Recognition, Assessment and Remediation Guidance American Industrial Hygiene Association Stock Number: EMRG10-764 ISBN: 978-1-935082-17-0

3 John Martyny

4 Connell CP, Sampling Strategies and Data Interpretation, Presented in Huntingdon, England (Nov. 2011)

5 Andersen, A. A. New sampler for the collection, sizing, and enumeration of viable airborne particles. J. Bacteriol. 76:471- 484 (1958)

6 Macher J. Burge HA, Sampling Biological Aerosols Chp. 22 in Air Sampling Instruments for Evaluation of Atmospheric Contaminants (ACGIH, 2001)BR>

7 Saulius T, Willeke K, Reponen T, Trunov M, Particle Cut-Size Evaluation –Final Report Nov 1998, Internal Report by Zefon International-Analytical Accessories, 2860 23rd Ave, St. Petersburg, FL, 33713

8 Lee KS, Black W, Brauer M, et al A Field Comparison Of Methods For Enumerating Airborne Fungal Bioaerosols, Presented at the Proceedings: Indoor Air 2002, Anaheim, California.

9 Reponen, T., Nevalainen, A., Willeke, K., Grinshpun, S. Biological Particle Sampling In: Baron, P., Willeke, K. Aerosol Measurement, Principles, Techniques, and Applications, 3rd ed. John Wiley and Sons (2001).

10 Jason Pohl The Denver Post, September 11, 2012

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