A slightly hazy view of the Eiffel Tower amid dense fog. (Jacques Brinon/AP)

Paris officials have suspended their order to keep half of the city's cars off the roads after a day of banning them because of thick smog and air pollution.

Cars with license plates ending in even numbers were barred from being driven on Paris-area streets Monday. Cars carrying three or more passengers, emergency vehicles, and electric and hybrid cars were exempt from the ban. Riders hopped on public transit for free.

The pollution reached high levels last week. The air was so bad that on Wednesday the city had a worse air quality index rating than even New Delhi and Beijing, according to Plume Labs, a company monitoring air quality in 60 international cities.

But officials say the day of cutting back traffic helped.

"Due to the improving situation today and tomorrow, the alternative traffic [plan] will not be renewed Tuesday," Ecology Minister Ségolène Royal said Monday.

Paris and its surrounding suburbs, with its more than 12 million people, regularly ranks among Europe's most air-polluted cities. The city tracks its air quality by measuring the concentration of particles in the air, or PM10, which are particles with a diameter of less than 10 microns (10 millionths of a meter).

[Map: These will be Europe’s most polluted cities by 2030]

Monday's ban came after Airparif, which measures the city's air, issued its highest alert when PM10 levels reached 80 micrograms per cubic meter, the Guardian newspaper reported. Friday's levels went above 100 mg per cubic meter, Airparif alerted. While 80 mg is considered safe, Paris's alert is set at 50 mg, and levels were expected to be between 40 and 50 on Monday.

The dry March weather contributes to the higher levels of dangerous particles filling the air. The city instituted a similar car ban in March 2014, and the Guardian reports that such emergency measures have been used only three times.

But there's also been some debate on Paris's tactics to combat its air pollution problem, and whether officials should instead focus on long-term solutions rather than one-day bans.

“Paris also needs a congestion charge inside the city,” said Fabrice Michel, a spokesman for the French Association of Transport Users. “This would reduce circulation and raise revenue. But all our politicians seem to do is wait for the rain, and when it doesn’t come, they blame the weather for their failings.”

City officials counter that they've invested heavily in public transportation. In December, the city prohibited open fire places, and trucks and buses polluting the air can cannot enter Paris starting this summer.

Others question whether it's accurate to measure a city's overall air quality by looking at just one day. “Air quality in the French capital is generally better than a decade ago,” Karine Leger, assistant director of Airparif, told France24.

Other cities with bad air quality have programs restricting road access. Under Mexico City's "Hoy No Circula" or "No Driving Today" program, diesel- and gasoline-powered cars with license plates ending with certain numbers are restricted from being driving on particular days of the week.


Motorists drive under a traffic board reading "Air pollution, slow down" near Rennes, western France, on March 21 , 2015. (Damien Meyer/AFP via Getty Images)