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The new EPA standard means 345 counties across the country have unhealthy air and will have to find ways to cut pollution. That’s four times more counties than violate the existing standard. Pollution from cars, power plants and factories combined with sunlight to create ozone, the primary component of smog. Babies and people with reduced immune systems and lung problems are most vulnerable. Ozone also reduces crop yields and forest growth. Many scientists and public health advocates believe the standard should be even tighter to protect public health and prevent premature death. EPA officials had planned to adopt a separate stricter standard to protect the environment. But they dropped that plan after pressure from the White House.