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Smoke & Ash Adulteration of Cannabis: Response #1

Steep Hill Labs appreciates the scientific Cannabis community at large and the media groups offering guidance and support to those affected by the recent California wildfires. We would like to illuminate certain topics of recent discussion, hoping to provide further details and encourage more community-sourced information sharing and education.

By: Lydia Abernethy

Does the use of Phos-Chek near my garden make my Cannabis unsafe?

The red slurry air dropped on the California fires of October 2017 was likely ICL’s Phos-Chek Fire Retardant, the only product approved for aerial application by the US Forest Service. ICL produces multiple forms of Phos-Chek; all are similar, chemical ratios varying by mode of application. Phos-Chek is applied to form a chemical fire break on threatened vegetation, helping to slow the progress of a fire. This fire retardant insulates and coats vegetation, restricting air flow and effectively reducing the availability of fuel sources.

Independent laboratories (non-Cannabis) have determined the retardants are not toxic by ingestion or dermal application. Respiratory sensitivity to dry-powder formulation of Phos-Chek has been documented; however, this dust exposure warning is not applicable once the product is in solution – as it is when applied on fires.
Phos-Chek solution loaded onto aircrafts is approximately 85% water. Ammonium phosphates, commonly used in agricultural fertilizers, are the main components of Phos-Chek; the FDA approves these chemicals as Generally Recognized As Safe (GRAS) when used in small amounts as feed additives for livestock and humans. Flow conditioner and gum thickeners used in this product are also GRAS by the FDA. The color pigments within Phos-Chek are considered visually unappealing, but are not hazardous. The chemicals used to control wildfires are not considered to be persistent or bioaccumulative. No adverse, long-term or chronic health effects have been attributed to the use of Phos-Chek.

A 2011 draft environmental impact statement released by the US Forest Service on aerial application of fire retardant states “impacts may occur on federally listed sensitive plant species and native plant communities” and may result in an “increase in noxious or non-native invasive plant species due to the fertilizing effects of aerially applied fire retardant.” It is estimated that 0.25% of retardant drops impact water; mostly thanks to avoidance area mapping, which discourages application within 300-feet of waterways (unless fire conditions require use near these sites to protect human lives).

Removal of wildland fire retardants is difficult after the product dries. Residues should be removed as quickly as possible; physical scrubbing or the use of surfactants may facilitate cleaning. It is not recommended to consume garden produce exposed to red slurry even after removing residues.

No studies have been conducted on the safety of consuming Cannabis tainted with fire retardants. If Phos-Chek directly contacted your garden, avoid releasing contaminated product to the market.


"Removal of wildland fire retardants is difficult after the product dries."

Is benzene detectable on Cannabis flowers via residual solvent analysis (RSA)?
Is this a relevant test for my smoke/ash-contaminated buds?

Benzene’s presence in the environment is ubiquitous due to natural and anthropogenic sources. It is detectable in a variety of produce and agricultural food commodities. Benzene is measured at high rates in cigarette and gasoline vapors. Half of the national personal exposure to benzene is from cigarette smoke. It is measurable in blood, with marked increase in persons exposed due to automobile-related activities, cigarette smoke, or occupational sources of emissions. Smokers are exposed to approximately 10 times the amount of benzene than non-smokers.

Natural and industrial sources release benzene into the atmosphere. Natural sources include crude oil seeps, forest fires, and plant volatiles. The release of benzene into the air by fire is monitored by government agencies worldwide. A 2001 study on Canadian municipal fires discovered a mean concentration of 3.45 ppm of benzene in atmospheric emissions.

The United States Department of Agriculture Forest Service released a study on exposure to air pollutants in smoke at wildfires among firefighters between 1992 to 1995 stating, “exposure to benzene and total suspended particulate was not significant.” Of all the fires studied for this report, only 3% of firefighter’s smoke exposures exceeded OSHA’s permissible exposure limits (PELs). Smoke exposure was generally greater among initial attack firefighters involved in active suppression. Additionally, exposure levels of benzene were higher among those using gas-operated machinery. Wind speed and direction affects these values. Many of the hazards encountered by firefighters can be avoided with proper hazard awareness training, smoke exposure monitoring, and limited respirator use.

Benzene is considered highly volatile. It is moderately soluble in water; this combined with the chemical’s volatility indicate that benzene readily partitions to the atmosphere. Precipitation and fog remove some benzene from the air. Large quantities of this chemical moved by rain to the ground will return to the atmosphere via volatilization. Benzene is highly mobile in soil and quickly leaches into groundwater. There is little to no evidence in available literature about the bioaccumulation of benzene in aquatic food chains. This is because benzene is readily degraded in water and soil under aerobic conditions; microbial communities facilitate the removal of this chemical from these environments. The presence of other aromatic hydrocarbons influences the rate of biodegradation. Vegetative contamination by benzene mainly occurs when the chemical is in the vapor phase via air-to-leaf transfer. Hattemer-Frey et al. discovered that the total concentration of benzene on food crops was detected in parts per trillion (ppt) due to air-to-leaf transfer (major) and root uptake (minor). Benzene accumulates in leaves and fruits of exposed crops. From research conducted in the 1990s, total concentration of benzene on exposed food crops consumed by humans was estimated at 587 ppt or 0.000547 ppm; this value is well-below the 1.0 ppm Action Level for Medical Cannabis Goods Meant for Inhalation and 2.0 ppm Action Level for All Other Medical Cannabis–Infused Goods from the now-retracted Proposed Medical Cannabis Testing Laboratory Regulations in California.

Studies on the presence of benzene on finished Cannabis flowers after municipal or wildland fires have not been conducted. Your lab of choice may not have established testing methods for residual solvents on Cannabis flowers, as that is an uncommon request. It is encouraged to discuss this test with your lab to determine if the results would yield potentially useful, scientifically valid data.

Steep Hill at the Fire Relief Fundraiser Organized by Harborside.

CW Analytical Laboratories. (2017, October 19). RSA on Smoke-Tainted Cannabis Flowers [Press release]. Retrieved from CW Analytical

Devitt-Lee, A. (2017, October 17). Fire, Brimstone & Dioxin. Project CBD. Retrieved from

ICL, Performance Products. (n.d.). PHOS-CHeK ® Fire Retardants For Use in Preventing and Controlling Fires in Wildland Fuels: Toxicological and Environmental Safety ICL’s Commitment to Safety [Press release]. Retrieved from ICL Fire Retardants

ICL Performance Products (n.d.). Safety Data Sheet PHOS-CHEK® WD881A Class A Foam Concentrate [Pdf]. Retrieved from

Reinhardt, T., & Ottmar, R. (2000, July). Smoke Exposure at Western Wildfires (United States, Department of Agriculture, Forest Service). Corvallis, OR: Forest Service Pacific Northwest Research Station. 1-84.

Tidwell, T. (2011, May). Aerial Application of Fire Retardant (United States, Department of Agriculture, Forest Service). Washington, DC: Fire and Aviation Management. 1-370. United States, Department of Health and Human Services, Agency for Toxic Substances and Disease Registry. (n.d.). Toxicological Profile for Benzene. 251-289. United States, State of California, Bureau of Cannabis Control. (2017). Bureau of Marijuana Control Proposed Text of Regulations, California Code of Regulations Title 16. Division 42. Chapter 5. Testing Laboratories (pp. 1-46). Sacramento, CA. Retrieved from