Multi-parameter characterization of indoor air quality in a medical marijuana cultivation facility in Oklahoma: a pilot study of occupational exposure determinants

The legal cannabis industry is expanding rapidly across the United States, yet indoor air quality (IAQ) characterizations of cultivation facilities remain scarce. Oklahoma's medical marijuana program grew to over 7,300 commercial licenses and 380,000 registered patients by 2023, but no published study has characterized occupational exposures within the state's cultivation facilities. Cannabis workers face poorly understood respiratory hazards, including elevated carbon dioxide (CO2) from plant respiration, particulate matter (PM) from cultivation activities, volatile organic compounds (VOCs), and extreme thermal and humidity conditions. This pilot study addresses this critical knowledge gap. We conducted continuous monitoring (10 July to 1 August 2023) in a 2,000 ft2 (approximately 186 m2) heating, ventilation, and air conditioning (HVAC)-equipped indoor medical cannabis cultivation facility in Central Oklahoma housing approximately 900 plants. The monitored room used an approximately 12-h light:12-h dark photoperiod, intermittent supplemental CO2, and a continuously operating single-zone HVAC system. Seven instruments were deployed across 3 spatial zones at breathing zone height (1.5 m): 3 Aranet4 nondispersive infrared (NDIR) CO2 sensors, 2 TSI AirAssure (8144) multi-parameter monitors (CO2, CO, NO2, O3, SO2, total volatile organic compound [tVOC], size-resolved PM), and 2 PurpleAir PA-II-FLEX dual-channel particle counters. Approximately 113,000 quality-controlled records were analyzed. CO2 concentrations exhibited a photoperiod-linked diurnal pattern driven by the plant light/dark cycle, with Aranet4 means ranging from 979 to 1,086 ppm and maxima exceeding 2,200 ppm. Elevated nighttime CO2 was consistent with the combined influence of plant metabolism, intermittent CO2 enrichment, and ventilation dynamics; worker respiration at night was unlikely to be the dominant explanation because workers generally left the room after lights were turned off. Median PM2.5 concentrations were low (1.4 to 3.1 µg/m3), but episodic spikes reached 5,000 to 35,000 µg/m3. The largest daytime PM excursion coincided with an in-flower canopy-maintenance event, which was trimming, defoliation, or canopy maintenance. Mean tVOC concentrations reached 3.61 mg/m3, and ethanol proxies exceeded 8,000 ppb at the 95th percentile. Temperature and relative humidity frequently reached levels that may increase heat stress and may also influence sensor performance under high-humidity conditions. This pilot study demonstrates that indoor medical cannabis cultivation facilities can present a complex, multi-hazard exposure environment involving CO2 enrichment and photoperiod-linked CO2 variability, episodic PM excursions, VOCs, and elevated heat and humidity. These findings highlight an urgent need for cannabis-specific occupational exposure research, facility ventilation guidance, activity-resolved monitoring, and worker health surveillance programs, particularly in states like Oklahoma where rapid industry growth has outpaced occupational health infrastructure.