The U.S. Environmental Protection Agency (EPA) on Wednesday proposed to strengthen air quality standards for ground-level ozone, the main component of smog, which has been linked to asthma, premature deaths and other health problems.

The proposal called for raising the country's National Ambient Air Quality Standards for ground-level ozone to a level within the range of 65 to 70 parts per billion (ppb), while taking comment on a level as low as 60 ppb.

The EPA said the proposed range was "based on extensive recent scientific evidence."

Under the Clean Air Act, the EPA was required to review the standards every five years and the agency last updated these standards in 2008, setting them at 75 ppb.

"Bringing ozone pollution standards in line with the latest science will clean up our air, improve access to crucial air quality information, and protect those most at-risk," EPA Administrator Gina McCarthy said in a statement. "It empowers the American people with updated air quality information to protect our loved ones -- because whether we work or play outdoors -- we deserve to know the air we breathe is safe."

Ground-level ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight.

Emissions from cars, trucks, buses, industries, power plants and certain fumes from fuels, solvents and paints are some of the major sources of NOx and VOC.

People most at risk from breathing air containing ozone include people with asthma, children, and those who are active or work outside. In the U.S., one in 10 children has been diagnosed with asthma.

According to the EPA's analysis, strengthening the standards to a range of 65 ppb to 70 ppb will prevent 750 to 4,300 premature deaths, 1,400 to 4,300 asthma-related emergency room visits and 65, 000 to 180,000 missed workdays.

The U.S. agency also estimated the proposed range will provide significantly better protection for children, preventing from 320, 000 to 960,000 asthma attacks and from 330,000 to one million missed school days.

The EPA stressed that the benefits of meeting the proposed standards will significantly outweigh the costs.

"If the standards are finalized, every dollar we invest to meet them will return up to three dollars in health benefits," it said. "These large health benefits will be gained from avoiding asthma attacks, heart attacks, missed school days and premature deaths, among other health effects valued at 6.4 billion to 13 billion dollars annually in 2025 for a standard of 70 ppb, and 19 billion to 38 billion dollars annually in 2025 for a standard of 65 ppb."

It estimated the annual costs at 3.9 billion dollars in 2025 for a standard of 70 ppb, and 15 billion dollars for a standard at 65 ppb.

Average ozone levels in the U.S. have fallen 33 percent from 1980 to 2013, the EPA said, adding depending on the severity of their ozone problem, areas would have between 2020 and 2037 to meet the proposed standards.

The agency will seek public opinions on the proposal for 90 days and plans to hold three public hearings before issuing final ozone standards in 2015.

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