Thursday, 6 May 2021 | The Latest Buzz for the Appraisal Industry

Illegal Indoor Marijuana Grow Operations – Part Two

Michael Cleveland
Michael S. Cleveland, President and Principal Scientist at Cleveland Environmental, Inc. and Mold Diagnostic Services.

This article originally titled, Recognizing Clandestine Indoor Marijuana Grow Operations for Property Insurance Claims and Property Management Professionals – Part Two of Three Parts, was originally posted on LinkedIn.

Part one of this article presented a real-life case study involving the landlord of an apartment complex who was sued for negligence and personal injury by a former tenant who claimed that water damage and mold in his apartment had exposed the tenant to toxic mold.

It was only after the vacant apartment was inspected by his defense attorney’s expert, a Certified Industrial Hygienist with a Master of Science in Public Health, who was an exposure scientist with expertise in moisture intrusion and mold issues in buildings, that the landlord learned that all of the extensive water damage, mold, and insect infestation in the apartment had been caused by a clandestine indoor marijuana grow operation. The expert had run into this type of situation many times before and knew how to recognize the telltale signs of a clandestine indoor marijuana grow operation, even when the operators tried to cover them up.

After the plaintiff’s attorney was told what the defense expert was going to say on the witness stand, the plaintiff’s attorney immediately dropped the case.

Clandestine indoor marijuana grow operations are an epidemic and they cause extensive damage to the buildings they are carried out in. In the words of the U.S. Drug Enforcement Administration in their report entitled, Residential Grows in Colorado, the New Meth Houses?, “Marijuana grows often cause extensive damage to the houses where they are maintained and are increasingly the causes of house fires, blown electrical transformers, and environmental damage. Much like the ‘meth houses’ of the 1990s, many of these homes may ultimately be rendered uninhabitable.”

Clandestine indoor marijuana grow operations are most often carried out in leased properties, including commercial spaces, residential homes, and apartments. Who would want to cause that kind of damage to their own property?

Clandestine indoor marijuana grow operations are proliferating for a number of reasons. It is not only because medical and/or recreational use has been made legal in some states. Clandestine indoor marijuana grow operations are prevalent across the United States and Canada as well.

Operators of clandestine indoor marijuana grow operations tend to ignore laws regarding indoor marijuana growing anyway. For example, while California and Colorado legally limit the number of marijuana plants that can be grown for recreational use to six per person, clandestine indoor marijuana grow operations usually exceed that number by far, and in some cases may contain hundreds of marijuana plants. The more plants they contain, the more property damage they cause.

One reason clandestine indoor marijuana grow operations are proliferating is that growing and selling marijuana is a profitable business. In 2019, the U.S. Drug Enforcement Administration and local authorities raided hundreds of black-market clandestine indoor marijuana grow operations across Colorado that were growing tens of thousands of plants and selling the marijuana out of state. More than 80,000 marijuana plants were seized after authorities raided 247 residences and eight commercial spaces.

Clandestine indoor marijuana grow operations are also proliferating because marijuana seeds and clones (young plants started from the leaves of mature marijuana plants with desirable strain qualities, such as high tetrahydrocannabinol [THC] concentration) are readily available commercially, even by home delivery in states where medical and recreational marijuana have been made legal.

A third reason that clandestine indoor marijuana grow operations are proliferating is that the information on how to efficiently grow marijuana indoors, and commercially-available equipment and supplies for doing, so have flourished on the internet (Google “grow marijuana indoors” and see what comes up!).

I categorize clandestine indoor marijuana grow operations into two types:

  • Type 1 – those that, once discovered, have obvious telltale signs that marijuana was being grown indoors (for example, a lot of marijuana plants still present)
  •  Type 2 – those that, once discovered, while having a lot of property damage, do not have obvious telltale signs that marijuana was being grown indoors (unless you know how to look for them) because the operators tried to conceal the evidence; it is the operators of Type 2s that, in my experience, may try to sue their landlords for negligence and personal injury for exposure to toxic mold

By the way, there is no such thing as “toxic mold” from an airborne exposure standpoint. Sure, there are case studies in the public health science literature where people who ate grain contaminated with certain species of mold became sick and died. Similar to what some species of mushrooms can do (ergo, “poison mushrooms”). But airborne mold spores cannot give you a “toxic” exposure. This has been well-established by public health science. Nevertheless, toxic mold is a myth that has reached urban legend status. I will be publishing another article on LinkedIn in the near future that will include a discussion on the origins of the toxic mold myth, and why “toxic mold” is a misnomer.

I have been retained many times now to provide forensic/expert witness support for property insurance claims/litigation involving both types of clandestine indoor marijuana grow operations. I encountered my first one about 10 years ago, a Type 1 in a 3,500 square foot, two-story residential home that had been leased by the operator for three years. The landlord didn’t have any idea what was happening in his property and had thought his tenant was great! The tenant was quiet, never complained, and always paid his rent on time and in cash.

Unbeknownst to the landlord, however, his tenant had been growing hundreds of marijuana plants inside of his property, using extensive equipment and supplies to so. The tenant had also made major alterations to the interior of the property, such as bypassing the electrical company meter to avoid paying for electricity, and cutting many round holes in ceilings and walls throughout the house to accommodate flex ducting used to exhaust heat from the many grow lights. Who knows how much longer it could have gone on for, but a grow light exploded and started a fire in the second-floor master bedroom.

It was a newer home with fire sprinklers in the ceilings and an alarm system linked directly to the fire station. The fire department arrived within minutes of the alarm, so the fire was quickly extinguished and contained to a corner of the master bedroom. The damage caused by the fire was, however, dwarfed by the extensive water damage and mold, and other property damage and safety/health hazards created throughout the house by indoor marijuana growing.

The operator had fled the scene before the fire department arrived and, as far as I know, was never seen again. He had used a false identity when he established the lease.

My assignment by the property insurance company to was to document all of the property damage so they could determine whether any of it was covered by the landlord’s insurance policy (which I understand turned out be $10,000 for water damage related to the fire, another blow to the landlord – it would cost many times that amount to remediate and repair the extensive property damage). When I arrived at the property, the police had removed the many grow lights as well as the top portions of the marijuana plants by cutting the plants at the base of their stems, leaving behind hundreds of one-gallon pots filled with soil.

The photo accompanying part one of this article is from that project. You can see several of the telltale signs of a clandestine indoor marijuana grow operation in the photo. While developing the report for that project, I did a lot of research on the growing of marijuana indoors and was quite fascinated on the art and science it had become. I also began to learn the many possible telltale signs, which helped me diagnose my first Type 2 as a clandestine indoor marijuana grow operation. Then I ran into another Type 2 on a lawsuit I was retained on to provide forensic/expert witness support. Then another…then another…and so on. It makes me wonder how many settled lawsuits there are out there involving Type 2s in leased properties where the defense never realized they were dealing with a clandestine indoor marijuana grow operation because the plaintiff-tenant successfully concealed what they had been doing.

So, the key to recognizing you are dealing with a Type 2 is to know the many possible telltale signs of a clandestine indoor marijuana grow operation. It is like solving a mystery. There will be an “ah ha” moment when you diagnose one. The more telltale signs you find, the more likely you are dealing with a Type 2. Clandestine indoor marijuana grow operations have become so common, I recommend that property insurance companies always consider the possibility when they receive claims involving water damage and mold, especially if there is no obvious source of the moisture intrusion.

I have found telltale signs of Type 2s in all of the various parts of lawsuit discovery. The inspection of the property by the defense expert is, of course, critical. What the inspection should include is described in part one of this article. At the site, the telltale signs of equipment and supplies used for growing marijuana may be found on the back patio or put away in closets, the garage or the attic, so be sure to check all spaces in the building and all areas of the yard as well. I have also found telltale signs in discovery casefile documents, photos, and depositions.

If you are fairly certain that you are dealing with a Type 2, but want more evidence to prove your case, you can also have your Certified Industrial Hygienist perform environmental sampling at the property. The Certified Industrial Hygienist can use collection media to wipe residue off smooth floors or vacuum dust from carpeted floors. Both types of samples can be analyzed in the laboratory for the presence of THC. The dust samples can be also analyzed in the laboratory by light microscopy. Under the microscope, marijuana plant particles found in the dust have a distinct appearance that can be definitively identified by the analyst.

Before I list out and describe the many possible telltale signs of clandestine indoor marijuana grow operations, let’s discuss remediation of properties impacted by indoor marijuana growing.

Two important authoritative references on the assessment and remediation of clandestine indoor marijuana grow operations are:

  • American Industrial Hygiene Association – Clandestine Indoor Marijuana Grow Operations: Recognition, Assessment and Remediation (2010)
  •  Canadian National Collaborating Centre for Environmental Health – Recommendations for Safe Re-occupancy of Marijuana Grow Operations (2009)

The former authoritative reference is available for purchase at the American Industrial Hygiene Association website:

The latter authoritative reference is available at no cost as a pdf at:

Both authoritative references describe the same approach for remediation of clandestine indoor marijuana grow operations. General steps for remediation include:

  1. First control any safety and health hazards that are immediately dangerous to life and health. Two such hazards can include combustion gases and electrical hazards. To increase carbon dioxide levels indoors, which promotes photosynthesis and the growth of marijuana plants, operators may disconnect the flues/exhausts of natural gas/propane gas appliances, such as hot water heaters, ranges, HVAC system furnaces and dryers, so they vent indoors. Natural gas and propane gas combustion products consist mainly of carbon dioxide gas and water vapor. But they also contain carbon monoxide gas, a deadly asphyxiant. Operators may also bypass the power company meters in buildings in a “jury-rigged” and possibly hazardous fashion to avoid paying for electricity, which is rapidly consumed by grow lights, axial fans used to promote marijuana plant stem health and ventilation systems used to exhaust heat from grow lights.
  2. Inventory any containers of fertilizers and insecticides and dispose of properly.
  3. Remediate water-damaged building materials and mold following applicable authoritative references (Environmental Protection Agency, Institute of Inspection Cleaning and Restoration Certification, etc.). This generally involves using a qualified remediation contractor to remove, under controlled conditions (HEPA-filtered negative pressure containment and remediation workers wearing personal protective equipment) porous moisture/mold-impacted building materials (drywall, insulation, carpeting, etc.) and cleaning and sanitizing remaining non- or semi-porous building materials (wood framing, concrete slabs, etc.) that are structurally sound. A thorough clearance inspection, including moisture meter testing of remaining building materials and air sampling for mold spores should be performed at the end of the remediation by a Certified Industrial Hygienist to ensure that all mold has been removed, airborne mold spores are within typical background levels and remaining building materials are dry enough for reconstruction. Some local public health agencies are now also requiring that clearance include surface sampling for THC to verify THC has been reduced to non-detectable concentrations.

Part three of this article will list out and describe the many possible telltale signs of clandestine indoor marijuana grow operations.

Make sure to keep an eye out on Monday for the final article of this series! Have any comments or would you like to submit content of your own? Email



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