In April 2018 researchers from China and the USA published the results of their review of the scientific literature to assess the association between air pollution and blood pressure levels. In particular, the association between particulate size, sulphur dioxide, nitrogen dioxide, nitrogen oxides, ozone and carbon monoxide and blood pressure levels were assessed. Results showed that a long-term exposure to particulate was significantly associated with risk of hypertension and that particulate plus nitrogen dioxide increased diastolic blood pressure. In addition, short-term exposures to particulate, sulphur dioxide and nitrogen dioxide were significantly associated with hypertension. A further analysis revealed a significant association between particulate, sulphur dioxide and systolic blood pressure, and particulate, sulphur dioxide, nitrogen dioxide and diastolic blood pressure. There was generally a stronger relationship in men, Asians, North Americans, and in areas with higher air pollution levels.