This study was conducted to estimate the risk of human cancer from the inhalation of formaldehyde in different indoor environments in Guiyang City in China. Offices had the highest mean formaldehyde concentration (0.11 mg/m³) and classrooms had the lowest mean formaldehyde concentration (0.04 mg/m³). The cancer risk levels in different indoor environments ranged from 6.96 × 10⁻⁶ to 2.48 × 10⁻⁴ and were greater than the acceptable cancer risk 1 × 10⁻⁶. High human formaldehyde risks imply a critical influence on human health in Guiyang City. The highest cancer risk values occurred in bedrooms (ranging from1.87 in 10,000 to 2.48 in 10,000). For office workers, offices were the highest risk environment after bedrooms where formaldehyde concentration levels were identified. Students were the most sensitive population, and the highest priority should be given to protecting their health in indoor environments. Regulations and standards for low emission materials and public education in relation to indoor pollution are highly recommended in China to protect human health.