San Jose's international airport was forced to close and several villages were evacuated after clouds of ash from the newly active Turrialba volcano reached the Costa Rican capital, authorities said.
Flights were suspended because of poor visibility at the airport, which is about 80 kilometers (50 miles) from the volcano, while pedestrians in the capital shielded their eyes to prevent ash going into them.
A number of schools sent students home and the country's emergency bureau ordered the evacuation of villages within two kilometers of Turrialba.
Turrialba erupted twice on Sunday, followed by a burst of activity Wednesday and then again on Thursday.
Before now, an expulsion of ash and magma from the volcano in October was its most significant activity in over a century.