Immigrants wave flags after being sworn in as U.S. citizens in naturalization ceremonies in Pomona, California.

By Stephanie Siek, CNN

(CNN) – The United States is becoming increasingly international, according to data collected by the U.S. Census Bureau  in the 2010 American Community Survey Thursday.

The number of foreign-born people in the United States is at an all-time high, at 40 million – an increase of about 9 million since the 2000 census. The 2010 census put the total U.S. population at almost 309 million.

But the foreign born comprise a smaller proportion of the total population (13%) than they did during the peak immigration years of the late 19th and early 20th centuries.

The 2010 American Community Survey also reveals that more than half of the nation’s foreign-born population lives in just four states: California, New York, Texas and Florida. Latin American immigrants remain the most numerous foreign-born group. Some 53% of all foreign-born population claimed their region of birth in Latin America and the Caribbean.

Just more than half of that group were born in Mexico, making it the most common country of origin – 29% of foreign-born residents, or 11.7 million people, were born there.

The next most common region of birth was Asia (28%), followed by Europe (12%), Africa (4%), North America (2%) and Oceania (less than 1%).

China was the second-most common country of birth for immigrants, with 2.2 million having originated from there. India and the Philippines tied for third, with 1.8 million each.Vietnam and El Salvador tied for fourth, with 1.2 million people from each country. Cuba and South Korea ranked fifth as countries of origin, with 1.1. million of the U.S. foreign-born population hailing from either.

Forty-four percent of foreign-born people are naturalized citizens, with European-born (62%) and Asian-born (58%) individuals having the highest rates of naturalization. People born in Latin America had the lowest rates of naturalization at 32%.

Citizenship rates increased the longer an immigrant remained in the U.S.  For immigrants who arrived before 1980, 80% are naturalized citizens. Among Asians who came to the country in the same timeframe, 90% are citizens.

By comparison, only about 13.7% of the total immigrants arriving since 2000 have become citizens, in part because immigrants generally must live in the U.S. continuously for five years as a permanent resident before qualifying for citizenship.