The United States on Thursday tightened its standards on smog -- also known as ground-level ozone -- but environmental groups said the anti-pollution measures don't go far enough to protect people's health.

The Environmental Protection Agency strengthened national air quality standards for ground-level ozone to 70 parts per billion (ppb), down from its previous level of 75 ppb.

"Put simply, ozone pollution means it hurts to breathe for those most vulnerable: our kids, our elderly and those suffering from heart and lung ailments," said EPA Administrator Gina McCarthy.

"Our job is to set science-backed standards that protect the health of the American people," she added, calling the new rule "one of the most important measures we can take for improving public health, reducing the costs of illness and protecting our children's health."

But environmental groups criticized the agency, saying it did not follow the recommendations of independent scientific advisors who had urged levels lower than 70 ppb, even as far down as 60 ppb.

"By setting a health standard that does not adequately protect Americans against harmful levels of smog pollution, President Obama has missed a major opportunity," said John Walke, senior attorney and director of the Clean Air Program at the Natural Resources Defense Council.

"Setting the safest recommended standard would have saved almost 6,500 lives and avoided nearly 1.5 million more asthma attacks per year than the smog pollution level the administration has chosen."

The Union of Concerned Scientists lead analyst Gretchen Goldman said the EPA chose "the most lenient rule possible given the agency's responsibilities to set the standard at a level that protects public health."

The price will be paid by the "the elderly, young children and those suffering from respiratory problems," Goldman added.

"Reducing ozone pollution is technically achievable and it's long overdue. But the EPA could have gone further in protecting the public by following the recommendations of scientists and public health experts."

Ozone forms in the atmosphere as a result of air pollutants like nitrogen oxides, which come from power plants and automobiles.

Ozone is also formed from organic compounds emitted by combustion and the evaporation of fuels, solvents and paints.

The EPA said its review of the evidence drew from "nearly 2,300 studies... including more than 1,000 new studies published since the last review of the standards in 2008."

The agency said the public health benefits of the updated standards are estimated at $2.9 to $5.9 billion annually in 2025, outweighing the estimated annual costs of $1.4 billion.