When talking about hemp, it is usual to hear that the crop has to contain less than 0.3% of THC in order to be considered lawful according to the federal regulations.  THC can refer to Δ9-THC, the phytocannabinoid having psychoactive effects, or it can refer also to the total amount of all the different THC isomers and the precursor of Δ9-THC, tetrahydrocannabinolic acid (THCA). THCA undergoes decarboxylation when treated with heat and it converts into Δ9-THC. The decarboxylation process can be induced also by time or exposure to light. Because of the statement of federal regulation refers to “THC” without any specifications, it is possible to interpret differently the law about THC content: it is possible to consider lawful a product having less than 0.3% of Δ9-THC by dry weight, or to consider the total amount of THC, included the potential Δ9-THC that can be formed upon decarboxylation of THCA. In order to calculate the potential amount of Δ9-THC, the percentage of THCA present is multiplied by the factor 87.7%, referring to the maximum conversion rate of THCA into Δ9-THC. This amount is then added to the percentage of Δ9-THC and the other THC isomers contained in the sample. Obviously when considering the total amount of THC, it will be always higher than considering only Δ9-THC %: where the regulation is interpreted considering lawful a crop containing less than 0.3% of total THC amount, there is less possibility of error in the THC levels in order to be compliant with the law. Because of the different interpretation of the federal regulations by local authorities, nowadays it is usual practice to include in the certificate of analysis (COA) of cannabis samples also the total amount of THC.