Fires at Indoor Marijuana Grows

June 1, 2017
Chris Jelinek presents common marijuana grow house hazards as well as response considerations for firefighters.

Firefighters face a seemingly endless list of situations that can result in their serious injury or death. Among these is an increased presence of indoor “grow houses” (i.e., residences used for marijuana production). These homes provide an increased risk at structural fires, and can result in property damage, injury and loss of life to both community members and firefighters. With this in mind, this article focuses on grow house hazards, recognition and response considerations for first responders.

Humboldt County’s unique situation

I have lived and worked in Northern California’s Humboldt County for more than 25 years. It is a beautiful place, surrounded by the natural beauty of majestic ancient redwood forests, wild rivers and lagoons, and sits adjacent to an unspoiled slice of the Pacific Ocean. Humboldt County, though extraordinarily scenic, is known for something else entirely—the prolific cultivation of cannabis in both outdoor and indoor marijuana grows. Up until a few years ago, marijuana existed in a counterculture-like presence within the community (you knew it was there, but few openly talked about it), but seemingly overnight, it evolved into something else entirely.

Marijuana grows are everywhere in Humboldt County. It is estimated that the county (population 135,000) contains more than 10,000 indoor and outdoor marijuana grows. Neighborhoods of all types have been infiltrated, and it is estimated that one out of every eight residences contains an indoor grow. No neighborhood is unaffected; residences within the most affluent communities to the least are impacted by indoor marijuana cultivation (IMC). And as a result, our firefighters have experienced the impact of these grow houses firsthand.

Grow house hazards

To understand grow house-related hazards, we need to begin with a basic primer on indoor cultivation techniques. Growing marijuana is really not that much different from any other type of plant. To thrive, plants need four elements: water, light, air and nutrients. These plants can either draw nutrients from soil, or be grown hydroponically (i.e., without soil, where nutrients are added to water pumped through a gravel medium). Light from household incandescent, florescent or LED lightbulbs is insufficient (they do not provide full-spectrum light needed for optimal plant growth), so high-pressure sodium and metal halide lights are used. These exceptionally bright lights are the same style used to illuminate gyms and baseball stadiums and parks.        

With each exhaled breath, mammals release carbon dioxide (CO2), which plants in turn use for photosynthesis. In an enclosed space such as a residence, available CO2 is depleted and needs to be replenished. This could be accomplished by simply opening a window to let in fresh air. This is problematic for growers, as marijuana has a very pungent odor, and open windows could lead to discovery of a clandestine grow. This problem is solved by either releasing CO2 into rooms via pressurized tanks or produced by burning propane, butane or natural gas. (Figure 1 shows an indoor grow room.)

An indoor grow house can be very dangerous. There are numerous hazards, including the presence of fertilizers and other chemicals, pressurized CO2 and propane cylinders, entanglement risks, structural modifications, overloaded electrical circuits and bypassed electrical meters. Here we’ll go through the problems associated with each of these hazards in greater detail. 


Chemicals are frequently present in indoor grows. These include plant fertilizers, herbicides, pesticides and fungicides, each presenting potential skin absorption and inhalation risks. Of particular concern is a device known as a sulfur burner (figure 2). This device utilizes a heating element to melt sulfur pellets, which in turn produces sulfur dioxide (SO2). SO2 functions as a topical fungicide to prevent powdery mildew from forming on the plants. SO2 released within an enclosed space presents a serious hazard. When SO2 combines with water, the reaction yields sulfuric acid (H2SO3). As our airway and lungs contain water, inhalation can result in internal chemical burns. 

Propane cylinders

Also concerning is the presence of pressurized CO2 cylinders and propane-powered CO2 generators. Increased CO2 within these tightly sealed rooms can displace oxygen, leading to asphyxiation. Further, propane cylinders and associated tubing has the possibility of leaking, resulting in an explosion risk. Brahe (2014) noted growers may deliberately vent gas furnaces and water heater exhaust fumes into these rooms. This not only releases CO2 but also carbon monoxide and sulfur dioxide, possibly leading to CO2 poisoning. Additionally, if exposed to fire, these metal cylinders could fatigue, and container failure could cause a boiling liquid expanding vapor explosion (BLEVE). 

Entanglement hazards

Numerous entanglement hazards exist. Among these are ventilation ducting, exposed electrical wiring, electrical cords, flexible plastic irrigation tubing, and rope (figure 3). Of particular concern is large-diameter flexible ducting, which is used in conjunction with inline ventilation fans for both intake and exhaust of air, heat and moisture from grow rooms (figure 4). This ducting, constructed of a lightweight aluminum foil laminate wrapped around small diameter helical wire, is notorious for entangling firefighters. Under fire conditions, the exterior sheathing burns/melts away, leaving the helical wire exposed. In a smoke-filled environment, these entanglement hazards can be difficult to detect.

Structural modifications

Structural modifications are common in grow houses. Holes are often cut into walls and ceilings to plumb ventilation ducting to the structure’s attic or exterior (figure 5). Under fire conditions, these penetrations could lead to rapid fire spread into attics and other concealed spaces. Due to high moisture content in grow rooms, rot damage may weaken structural elements. Load-bearing walls could be removed to make way for cultivation, leading to structural compromise. In extreme cases, holes have been cut through joists and floorboards to allow plants to be grown directly in soil beneath the subfloor.

Marijuana growers also labor to conceal their efforts. Marijuana has a distinctive skunk or rotten cabbage-like odor. To address this, growers use charcoal filters (figure 6) to remove the odor prior to exhausting air. In addition, these rooms are commonly sealed with plastic to prevent odors from escaping. This plastic (or plywood in some cases) is also used to keep very bright grow lights from being visible on the home’s exterior. In some cases, false walls are built in front of windows, and furniture is placed in the space between the false wall and windows to give the appearance of an occupied residence. In one unusual case, the Arcata Fire Protection District responded to a structure fire and located a grow room in which the interior entrance was concealed behind a sliding bookcase (figure 7).

Electrical hazards

One of the most significant indoor grow-related hazards is the potential for electric shock. Due mainly to lighting needs, IMC residences use an astounding amount of electricity. Each high-pressure sodium/metal halide light uses between 500 and 1,000 watts each, with a typical residential grow containing anywhere from 10 to 40 overhead lights (figure 12). One of these homes can consume upwards of 10,000 to 12,000 kW hours of electricity per month, nearly 20 times average household use! In addition, electrical equipment may be poorly wired and the home’s circuits overloaded, leading to potential shock and fire ignition sources. 

Of great concern are bypassed electrical meters. Due to either fear of detection by electric utility company employees or in wanting to avoid paying high electricity bills, growers will bypass electrical meters. As a result, personnel at an IMC fire may secure electrical utilities only to discover there is still power to the residence. If responders make contact with exposed wiring or spray hose streams onto energized equipment, there is a risk of electrocution.

Grow house recognition

Fortunately, there are some visual, auditory and olfactory cues to help firefighters recognize marijuana grow houses. Although these grow house “tells” are not present in every case, they can be useful in recognizing the hazards a home may possess. These indicators include:

  • A distinctive “skunk” or “rotten cabbage-like” odor of marijuana emitting from the residence
  • Unusual sounds coming from inside the residence (these are caused by grow light ballasts and sound similar to the humming of a utility pole-mounted transformer)
  • Bright light emitting from cracks around windows and doors
  • Plastic sheeting, plant stocks, fertilizer bags, plastic piping and large amount of soil and pots observed in the trash
  • Large quantities of potting soil and lumber delivered to the residence
  • Condensation collecting on windows (caused by high humidity in grow rooms)
  • Blacked-out or heavily shaded windows that are never opened
  • Windows and doors that are fortified with security bars not in character with the neighborhood
  • Extra locks on doors or gates that normally wouldn’t require it
  • The absence of snow or frost on roofs as compared to similar homes or rooflines, (figure 9) caused by the heat produced from grow lights

Many of the abovementioned cues can be observed during district familiarization. As you are driving or walking through your first-due, be observant for these indicators. This information can be useful in making go/no-go decisions for interior operations if the building were to catch on fire. Unfortunately, these indicators are not always present, and discovery of a marijuana grow may not occur until crews are actively engaged in firefighting operations.

Response considerations

As with any incident, begin by gathering intelligence, identifying hazards and conducting a risk-benefit analysis. During your 360, make note of any hazards. Prior to deciding on your strategic approach, try to answer the following questions:

Is a marijuana grow room involved with fire or has a separate uninvolved grow room been discovered? For example, suppose fire attack locates and extinguishes a kitchen fire, while separately a search crew locates an isolated marijuana cultivation operation in the garage. Under those conditions, you may be able to isolate that room, communicate its presence and associated hazards to the incident commander (IC), and continue with interior operations. If, on the other hand, you discover a residence used entirely for IMC, or grow rooms are involved with fire, use this information as a trigger point for a defensive operation.

Can electrical utilities be secured? Regardless of the scale of an indoor grow, the inability to secure a building’s electrical utilities is a huge red flag—one warranting a defensive approach.

Is this an occupied or unoccupied structure? Some residences may be used exclusively for marijuana cultivation, whereas others may be occupied. In Humboldt County, where marijuana cultivation and use is fairly normalized, it is not uncommon for grow houses to be occupied by families. With reliable indicators of trapped occupants, assuming there are survivable conditions and electrical hazards can be mitigated, consider an offensive approach.

Have entanglement hazards been encountered? If interior crews discover entanglement hazards, they should immediately disengage and communicate the hazards to the IC. Detecting entanglement hazards is yet another trigger point for transition to defensive operations. If firefighters become entangled and cannot immediately extricate themselves, a mayday should be declared. Because of these hazards, I recommend issuing wire cutters to all firefighters and ensuring they are trained in their use under reduced visibility conditions.

The bottom line here: If you have any doubt as to the presence or severity of hazards, go defensive. No indoor marijuana grow is worth a firefighter’s life!

Consider the following guidelines when responding to marijuana grow houses:

  • As soon as evidence of IMC is observed, ensure that all responding personnel and the dispatch center are notified over the incident’s command and tactical channels.
  • Broadcast an emergency alert tone notifying personnel of the hazards.
  • Request the electrical utility company respond to verify the building’s power has been isolated.
  • Using the abovementioned criteria, establish either an offensive or defensive strategy.
  • Following fire knockdown, continue to wear SCBA during overhaul operations. Due to the presence of fertilizers, herbicides, pesticides and sulfur emitters, use SCBA to reduce inhalation exposures.
  • Notify law enforcement for investigative follow up of possible criminal activity. Preserve related evidence and communicate your findings to fire investigators.

In sum

With ongoing nationwide efforts to legalize medicinal and recreational marijuana, it is likely that IMC-related fires will continue to occur. Recognizing their presence within your community, understanding related hazards, and developing agency adopted operating guidelines are steps you can take to ensure these incidents are mitigated in the safest way possible. 


Brahe, C. Marijuana grow rooms becoming common. 2014. Retrieved from Colorado Inspection Services LLC:

About the Author

Chris Jelinek

Chris Jelinek has been in the fire service for more than 25 years and currently serves as a battalion chief for the Humboldt Bay Fire Department in Eureka, CA. He has previously served as an associate faculty member for College of the Redwoods’ Fire Technology Program, president of the Humboldt County Training Officers Association and as president of the Northern California Training Officers Association. He has bachelor’s degree from Humboldt State University and is a graduate of the National Fire Academy Executive Fire Officer Program.

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