Funding for this research was provided by:
National Basic Research Program of China (2012CB955504)
National Major Research and Development Program (2016YFC1200802)
China Prosperity Strategic Programme Fund (15LCI1)
Received: 20 March 2019
Accepted: 27 December 2019
First Online: 8 January 2020
Ethics approval and consent to participate
: Ethical approval for the study was obtained from the Chinese Center for Disease Control and Prevention Ethical Review Committee (No. 201214). All data analyzed were anonymized.
: This study has not been published previously or under consideration for publication elsewhere. The authors consent to publish the study in Environmental Health.
: The authors declare that they have no competing interests.