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The Port of San Diego contributes to city's failing grade, from diesel-burning ships to trucks that move goods on and off those ships. But the number one contributor to unclean air is the cars on the road.

"The emission from the cars gets cooked by the sun and they turn into ozone," said Debra Kelley, who is with the American Lung Association.

It produces gases that burn the delicate tissues in the lungs.

In a new report released by the American Lung Association, San Diego was ranked the seventh worst in the country when it comes to air quality. Los Angeles was ranked number one.

"Here in California, air pollution kills more people than breast cancer," said Kelley.

The American Lung Association said San Diego's air quality can create health conditions like asthma, or worsen existing illnesses.

Carey Poindexter said he cannot leave the house on days when the air is really bad.

"Each time you take a breath, it feels like you're breathing through a straw," he said.

The 10-year-old carries a bag of medicine with him at all times to prevent an asthma attack.

"This bag here is filled with my medication that I take every day," he said. "I take seven medications every day."

Despite flunking standards set by the American Lung Association, one government agency says the air in San Diego is just fine.

The San Diego County Air Pollution Control District calls grading used by the American Lung Association "unscientific" and told 10News it would give San Diego a "B" following tough standards set by the Environmental Protection Agency in 2008.

Even the American Lung Association agrees the air is getting better in San Diego, mainly because people are driver greener cars like hybrids.

Some improvements also come from part of the port, where ships can now plug into electricity off the city's grid.

The American Lung Association said the military contributes to air pollution in the Port of San Diego.

Even without pollution from Los Angeles and Mexico, the American Lung Association said San Diego would fail its standards.