Therefore, in actual disposal situation, the solidified/stabilized fly ash, which is initially classified as posing a low risk level before landfill disposal, may transform with time after exposure to increasingly acidic conditions, such as increased proton activity caused by landfill microbes, atmospheric CO2, acid rain, and/or severe long-term soaking/leaching of acidic leachate in MSW landfills, and hence, pose high or very high levels of risk. Additionally, future studies should focus on further leaching experiments considering the real-world landfill environment, which can help to further study the leaching behavior and environmental risk of heavy metals in MSWI fly ash under complex landfill conditions.