In recent years, there has been some discussion around air quality and if/how commercial cannabis cultivation may contribute. Terpenes, while conferring many benefits under normal circumstances, are volatile organic carbons (VOCs). They may react with hydroxyl (OH–) or nitrate (NO3) radicals in certain environments and generate harmful byproducts.  Thus, a 2019 study published in Atmospheric Environment determined “to estimate the emission capacity range and terpene emission composition of cannabis plants.”
The key concern is ozone (O3) — the protective layer in the upper atmosphere (the ozone layer) but potentially injurious near the ground (smog). Given certain reactions—including with O3—terpenes may indirectly help generate O3 and byproducts including formaldehyde. The researchers noted that Denver, Colorado, is home to over 600 licensed cultivation facilities. Released in huge quantities, they believe terpenes from mass cultivation may negatively impact air quality. No actual impact has been recorded. This study aimed to begin by measuring the terpene emissions from live plants.
Specifically, they looked at four cultivars: Rockstar Kush, Elephant Purple, Lemon Wheel, and Critical Mass. At different growth stages, the largest, tallest plant of each cultivar was placed into a Teflon pail liner. Ambient air pumps allowed the researchers to circulate air at a constant rate for 30 minutes; two adsorbent columns with Tenax TA and Carbograph 5TD attached to the lines flowing out of the enclosure. Samples were thus collected then analyzed using gas chromatography with mass spectrometry and flame ionization detection.
Perhaps not surprisingly, the authors noted that “composition of terpene emissions varies among the growth stages and [cultivars].” Critical mass was markedly higher in emission rate at both 30 and 46 days. Respective emission rates, reported as micrograms (μg) per hour (h), were 1.4 and 8.6 for Critical Mass; 0.9 and 4.5 for Elephant Purple; 0.9 and 3.5 for Lemon Wheel; and 0.7 and 2.2 for Rockstar Kush.
The dominant terpenes across cultivars were β-myrcene and eucalyptol although numerous differences were observed. Critical Mass, for example, emitted mostly β-myrcene (43%) at 30 days, but by day 46, the dominant terpenoid was eucalyptol (32%) and myrcene content had declined (18%). The number of terpenes detected increased from 30 days to 46 days across cultivars. By day 46, Elephant Purple and Rockstar Kush tested for 17% and 8% limonene, respectively, in addition to notable quantities of caryophyllene, α-pinene, and others.
The authors calculated the potential of the plants to form ozone and concluded that “If these emissions from these cultivations are released into the ambient atmosphere, they have the potential to impact local ozone and particulate matter.” They acknowledge that there are at least 600 distinct cultivars across facilities in Denver, Colorado, meaning that the emissions from these four cultivars can’t be generalized to represent all cannabis. Additionally, the study’s growing environment forced “field measurements” due to lack of control. 
Terpenes are widely beneficial—in this case, emissions would be reacting with pollution in urban air. Vast numbers of flowering plants aren’t typically grown intensely in cities. The question warrants further scrutiny and perhaps an innovation or technological solution. Cleaning up the air wouldn’t hurt, either.
- Wang CT, et al. Potential regional air quality impacts of cannabis cultivation facilities in Denver, Colorado. Atmospheric Environment. 2019;199:80-87. [Impact Factor: 4.039; Times Cited: n/a]