d-Limonene can downregulate the expression of fibrotic markers such as COL1A1, COL3A1, and α-SMA and can reduce the content of TGF-β1 and VEGF mRNA in tissues, which is of positive significance for COVID-PF and even ARDS in the middle and late stages. The gene discussed is COL3A1; the disease is acute respiratory distress syndrome.