Emergency teams responded to the Pine Ridge Care Center around 11:15 a.m. Tuesday after a work crew spilled a cleaning product containing sulfuric acid.
Eight residents suffered minor breathing problems and were taken to the hospital.
"They might have some suppressed immune systems already or a medical state where they need some extra care, so it's more precautionary, they had minor respiratory irritations," emergency service manager Steven Hancock said.
A hazardous materials team from the San Rafael Fire Department spent the afternoon cleaning up the spill.
Authorities gave the all clear to reopen just before 3 p.m.