Does Light Break Down THC?

Since the days of Edison and Tesla, light has become a part of everyday modern life. In fact, it’s so common that we tend to forget what light is. Light is radiation, specifically electromagnetic radiation within a certain spectrum. Radiation degrades organic material when sufficient energy is transferred. This is evident to anyone who’s gotten sunburn, but is also the same process that turns grass brown and fades untreated wood. The irony is that light is also an important part of the growing process, especially when it comes to cultivating high-THC strains. The more UVB light a growing cannabis plant receives, the more potent its buds will be. On the flip side, once cannabis has been harvested, exposure to light will promptly deplete THC content because of the biosynthetic processes that occur at the molecular level in response to that exposure. Most visible light is non-ionizing radiation, and thus, generally harmless to humans and living things (including cannabis).

The complicated part lies in ultraviolet (UV) radiation. Ultraviolet light sits right along the line between ionizing and non-ionizing radiation on the electromagnetic spectrum. The difference is so close that UV light is further subdivided, with the lower half of UV referred to as “soft” UV, and the upper half considered ionizing radiation. Due to the unique properties of UV light, some of the non-ionizing spectrum is even able to exhibit ionizing-like effects. Multiple studies have shown ultraviolet light to be a primary contributing factor to THC breakdown.

Interestingly, the very elements that can degrade THC also catalyze the biosynthesis of THC to begin with. Raw cannabis does not contain any THC—it contains THCA, the acidic and inactive precursor to the psychoactive cannabinoid. Decarboxylation is the process by which a combination of heat and time cause the cannabinoids to lose a molecule of carbon dioxide. That synthesis transforms each cannabinoid into something slightly different than it was before. THCA, a therapeutic but non-cerebral molecular structure, becomes THC, the psychoactive compound for which pot is most well-known.

Many light sources, including the popular Compact Fluorescent Lamp (CFL) bulb (the type one might find in most commercial businesses, including a dispensary), emit a small portion of UV light (sunlight that reaches earth also carries a small portion of UV, typically more than commercial bulbs). As THC is a relatively unstable compound, only a small amount of light is required for degradation. For this reason, cannabis light exposure should be minimalized wherever possible. If your local dispensary keeps their stock in large clear jars, it might be wise to look elsewhere.

Time and Temperature

The exact temperature and duration of time by which decarboxylation occurred remained mysterious. In 2016, a study published in Cannabis and Cannabinoid Research sought to fill in that gap by investigating the most effective decarboxylation methods for the synthesis of acidic cannabinoids into their activated forms. The study found that decarboxylation was complete (all of the THCA was converted to THC) after THCA was heated at either 110 degrees C for 30 minutes; at 130 degrees C for 9 minutes; or at 145 degrees C for 6 minutes. The study also found that after decarboxylation, the overall content of THC also decreased, indicating that as soon as THCA has been converted to THCA, it begins to deteriorate.


The process of oxidation occurs when a chemical reaction causes the loss of electrons in a molecule, atom, or ion. Oxygen is not the only agent that can cause oxidation, but when it comes to THC degradation, that’s what we’re talking about. In practical terms, air flow is a contributing factor in the degradation of THC. When THC is oxidized, it converts to cannabinol (CBN), a useful, but non-psychoactive and much less potent cannabinoid.

A high content of CBN in a cannabis strain indicates a lot of THC degradation, so don’t expect to get a very potent high from it. However, CBN is still a sought-after cannabinoid for cannabis consumers who are looking for ways to enhance their wellness. CBN is a powerful sleep aid. In fact, CBN is the primary reason why cannabis can be such an effective treatment for insomnia. It has also demonstrated potential as an antibacterial, analgesic, anti-inflammatory, appetite stimulant, anti-convulsive, bone stimulant, and glaucoma treatment. So if your cannabis is stale, hold off on throwing it out—it might still be useful if not particularly recreational.

What Evidence Do We Have?

A few studies have delved into the degradation of THC. THC itself is not a very stable compound and will degrade into CBN with exposure to light, air and heat. A 1976 study determined light to be the largest factor of the three. A more recent four-year study delved deeper into the phenomenon, finding “on the average, the concentration of THC in the plant material decreased by 16.6% ±7.4 of its original value after one year, 26.8% ±7.3 after two years, 34.5% ±7.6 after three years and 41.4% ±6.5 after four years,” for cannabis stored at room temperature.

Researchers found that “the higher the concentration of THC, the faster the degradation over the first one or two years.”
Additionally, “the degradation of THC appears to proceed at a higher rate for the first year than subsequent years and levels off after two years to a rate of loss of approximately 7 % per year.” At a loss rate of roughly 1.3% per month (and a wide degree of variance), the majority of cannabis smoked within a year of harvesting should be relatively close to ideal freshness.

How to Properly Store Your Cannabis Stash

Cannabis is stored best when placed in an airtight, humidity-controlled, dark, cool location. Packing cannabis in a tinted mason jar and placing that jar in a dark closet is the cheapest route to take. Heat should not be a problem so long as you are keeping your bud at room temperature, but maybe don’t store it near the radiator or heating vent. Freezing is technically optimal, and works well for long-term storage. Trichomes may become brittle and fall off, but if kept away from light and air, should avoid cannabinoid degradation. Ultimately, there are many ways to properly preserve your stash so try a few options out and pick what’s best for you!