Risky smog levels have prompted a second air quality alert for the Bay Area this week as the forecast shows high temperatures and low winds.

Those weather conditions will combine with car and factory emissions for increased smog levels that could last through Wednesday and Thursday, the Contra Costa Times reported.

Area officials have issued a "Spare the Air" alert for Tuesday, encouraging people to cut back on driving and to limit outdoor exercise.

"Vehicle traffic accounts for most of the ozone-forming pollution in the Bay Area, and choosing commute alternatives such as carpooling or taking transit can make a big difference when it comes to protecting air quality and our health," air district spokesman Jack Broadbent said in a statement quoted by NBC's Bay City News.

The first Spare the Air alert for the season was issued by the Bay Area Air Quality Management District for Monday, followed by Tuesday's advisory.

The high smog levels seem to be part of a country-wide trend as pollution increases in America's most-populated areas.

The American Lung Association recently released its annual study on national air quality, revealing that some 148 million people inhabit areas that have unhealthy amounts of pollution in the air, The Guardian reported.  

According to the new project, about half of Americans are breathing air with unhealthy levels of pollution due to soot and smog particles on a daily basis.

Risky levels of ozone and particle pollution can make people cough or wheeze and can result in asthma attacks, heart attacks and premature death. Those at the greatest risk include infants, children, teenagers and older adults as well as people who have lung diseases such as asthma, heart diseases or diabetes.

In cities like Beijing, China, pollution has reached such dangerous levels that outside activity is greatly limited on particularly bad days and driving has been restricted.

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