**Seulkee Heo \*, Chris C. Lim and Michelle L. Bell**

School of the Environment, Yale University, New Haven, CT 06511, USA; chris.lim@yale.edu (C.C.L.); michelle.bell@yale.edu (M.L.B.)

**\*** Correspondence: seulkee.heo@yale.edu

Received: 12 September 2020; Accepted: 9 November 2020; Published: 12 November 2020 -

**Abstract:** Human mobility is a significant factor for disease transmission. Little is known about how the environment influences mobility during a pandemic. The aim of this study was to investigate an effect of green space on mobility reductions during the early stage of the COVID-19 pandemic in Maryland and California, USA. For 230 minor civil divisions (MCD) in Maryland and 341 census county divisions (CCD) in California, we obtained mobility data from Facebook Data for Good aggregating information of people using the Facebook app on their mobile phones with location history active. The users' movement between two locations was used to calculate the number of users that traveled into an MCD (or CCD) for each day in the daytime hours between 11 March and 26 April 2020. Each MCD's (CCD's) vegetation level was estimated as the average Enhanced Vegetation Index (EVI) level for 1 January through 31 March 2020. We calculated the number of state and local parks, food retail establishments, and hospitals for each MCD (CCD). Results showed that the daily percent changes in the number of travels declined during the study period. This mobility reduction was significantly lower in Maryland MCDs with state parks (*p*-value = 0.045), in California CCDs with local-scale parks (*p*-value = 0.048). EVI showed no association with mobility in both states. This finding has implications for the potential impacts of green space on mobility under an outbreak. Future studies are needed to explore these findings and to investigate changes in health effects of green space during a pandemic.

**Keywords:** mobility; vegetation; green space; sustainability; social media; disease prevention
