Two of Italy's largest cities, Milan and Rome, are restricting car use as smog levels build up.
Milan is banning cars, motorcycles and scooters for six hours a day over the next three days.
In Rome, cars with odd-numbered plates have been banned for nine hours on Monday. On Tuesday, cars with even-numbered plates will be restricted.
Experts say unusually calm and dry weather means that pollution is not being dispersed.
In Rome, cars deemed to be environmentally friendly, such as those with hybrid engines, are exempt from the ban.
Milanese authorities have introduced a special "anti-smog" all-day public transport ticket for €1.50 (£1.05; $1.65).
The ban there will be in force for six hours a day until Wednesday, with drivers facing fines if they do not comply.
Announcing the ban last week, Milan Mayor Giuliano Pisapia appealed to all the city's municipalities to observe the three-day ban.
"In these days of major emergency, we cannot remain indifferent," he said in a statement (in Italian).
What is smog?
Smog is a type of pollution involving fine particles less than 2.5 microns (0.0025mm) in diameter. It has been linked to lung damage and respiratory illnesses.
In 2012, Italy had the most pollution-related deaths in Europe. Over 84,000 people in the country died prematurely owing to bad air quality, according to the European Environment Agency (EEA).
Milan was named as Europe's most polluted city in 2008 and it remains among the worst on the continent. City officials have limited traffic on several occasions in the past, first trying out a ban in 2007.
The capital Rome has limited traffic on several occasions.
Two major Spanish cities have also imposed measures to reduce pollution. A 90km/h (56mph) speed limit was introduced in the Barcelona area last week and parking for most vehicles has twice been banned from the centre of Madrid since last month.
Pollution is also affecting other aspects of Italian life. Earlier this month, the mayor of San Vitaliano, just outside Naples, banned the use of wood-fired pizza stoves.
Under the edict, the stoves need to be fitted with special pollution filters before they are allowed back into action.
The Chinese capital Beijing has introduced similar restrictions in recent days after some of the worst smog in the city's history. The city last week declared a red pollution alert - the most severe of its kind - for the second time this month.
The restrictions in Beijing mean cars can only be driven on alternate days, depending on whether their number plates end in an odd or even number. Statistics show that about 112,800 vehicles violated the rule in just four days, according to Beijing News.