PARIS — Parisians taking public transportation to work on Friday were surprised and delighted to find free subways and buses for the next three days, but the reason was a bit less cheerful: Air pollution had reached an unusually high level and was expected to continue unabated through the weekend.

Bad air quality might be familiar to tourists here from Beijing or Mumbai, but in this elegant capital, where strict limits on building height create the illusion that plenty of fresh air is circulating, pollution is rarely this severe so early in the year and for so long.

“Due to a persistent episode of pollution with fine particles,” the Environment Ministry will impose exceptional measures, said Philippe Martin, the minister, in a somewhat cryptic statement on Thursday.

His widely reported message left some people a little unsure of exactly what he meant.

The French Health Ministry issued warnings particularly for the elderly, pregnant women, young children and those with respiratory ailments. In the parks, joggers could be heard complaining that they were finding it more uncomfortable than usual to run.