WASHINGTON: The US population will hit the historic milestone of 300 million on Tuesday but no one can say who or where the person who tips the scale will be on that day.

According to the US Census Bureau, the 300th million resident will clock at about 7:46 am (1146 GMT) based on an algorithm that takes into account birth rates, death rates and rates of international migration.

The Census Bureau estimates that an American is born every seven seconds; one dies every 13 seconds and a migrant crosses into the country every 31 seconds. The result is an increase in the total population of one person every 11 seconds.

A spokesman for the Census Bureau said that with that formula in mind, it is near impossible to determine whether the 300th million resident will be a newborn, an immigrant coming across the border or someone flying into the United States.

"It would be very difficult to say who the 300th million person is just because of the number of people that move in and out of the country and also because of the range of births that take place," Stephen Buckner told AFP.

He said that no special celebrations were planned for Tuesday's milestone, which makes the United States the third most populous country in the world after China and India and which comes almost 39 years after the 200 million benchmark was reached on November 20, 1967.

At that time Lyndon B. Johnson was president, colour television was the rage and the country was preoccupied with the Vietnam War.

Today's America under President George W. Bush is mad about iPods and is engaged in wars on two fronts -- Afghanistan and Iraq.

Illegal immigration, mainly from Latin America, is nowadays also a hot-button issue being played up in upcoming legislative elections in November.

According to the Census Bureau, 14 percent of the current US population is Hispanic, compared to four percent in 1968, and it is projected that 25 percent of the population will be Hispanic in 2050.

It is also expected that in the next 50 years there will be more Hispanic births in the United States than immigrants.

William Frey, a demographer at the Brookings Institution in Washington and a professor at the University of Michigan, said the changing ethnic makeup of the United States and the new immigrants should be welcomed, rather than scorned, as they represent the future of the country.

"What the new population growth represents is a return to our immigrant past," he said.

Buckner said the 300 million landmark was a momentous occasion for the nation that all Americans should look upon with pride.

"This is a point in time at which people maybe reflect on what type of changes have taken place over the time that we've been growing as a nation," he said.

Many, however, are using the occasion to sound the alarm about the environmental impact of the growing population, which is expected to reach 400 million by mid-century and 600 million by the end of the 21st century.

"I think the 300-million mark is reason to celebrate a great nation," said Vicky Markham, director of the Centre for Environment and Population, based in Connecticut. "But a nation isn't great if its citizens can't take a critical look at who they are and how they are a player in the global context."

She said although the US represents only five percent of the world's population, it has the "largest ecological footprint in the world".

"The US is the only industrialised nation in the world with significant population growth and that, combined with our rate of resource consumption, result in America having the largest environmental impact in the world," Markham said. - AFP/so