In 2010, Boston was estimated to have 617,594 residents (a density of 12,200 persons/sq mile, or 4,700/km2) living in 272,481 housing units— a 5% population increase over 2000. The city is the third most densely populated large U.S. city of over half a million residents. Some 1.2 million persons may be within Boston's boundaries during work hours, and as many as 2 million during special events. This fluctuation of people is caused by hundreds of thousands of suburban residents who travel to the city for work, education, health care, and special events.