Many areas of the Czech Republic have witnessed high levels of smog due to unusually high levels of airborne particles. Smog levels have been high since late last week. The city of Prague declared a smog emergency last Thursday and is now considering free public transport in an effort to limit traffic in the capital. The air quality has been poor in other regions as well.
As a consequence, people with chronic respiratory conditions, heart problems and young children as well as the elderly should reduce the time spent outdoors. Healthy people should avoid doing physically demanding activities as well. Unnecessary driving should be also avoided as it contributes directly to the air quality.
A smog emergency is declared when the amount of PM10 airborne particles exceed the limit of 100 micrograms per cubic meter for longer than twelve hours and do not drop below that limit for 24 hours.
You can check the up-to-date information regarding air quality in different regions of the Czech Republic here: http://portal.chmi.cz/files/portal/docs/uoco/web_generator/actual_hour_data_GB.html
UPDATE 24.1.2017: The smog emergency warning has been lifted in Prague today. In some areas the air quality is still bad.