Harvard University 6 cities cohort study established the association of PM_2.5 and exposure and mortality association. In 2005, WHO introduced Air Quality Guideline of PM_2.5, and the Global Burden of Disease ranks air pollution as the second risk factor for global premature deaths. Comprehensive evidences of epidemiological and biological mechanisms have established that air pollution can affect almost all organs of our body. However, the health effects of PM2.5 has a complicate nature, determined by factors such as time after exposure, size distribution and chemical compositions of PM_2.5, as well as susceptibility of subgroup of exposure population. By integrating ambient, personal, and internal exposure with clinic and multi-omic methods, panel studies can provide the evidences of the health effects and mechanism of size distribution (e.g. UFP) and chemical compositions (e.g. BC, Organics, Metals) of PM_2.5, on the health of susceptible subgroup (e.g. individuals or with prediabetes, CVD, COPD). In the last decades, air quality improvement in the developed countries and China have achieved great health benefits, saved millions of lives. In 2025, the 2^nd WHO Global Conference on Air Pollution and Health concludes with ambitious commitment to protect global public health to reduce mortality attributed to air pollution by 50% in 2040. To achieve this commitment, joint efforts of clean air actions and climate mitigation are needed.