If you visited or lived in the Los Angeles area many years ago, you why it was called, unofficially, the City of Smog. In the early 1970s, I attended the LA Zoo with my third grade classmates on one of those given days when the South Coast Air Quality Management District likely issued a smog warning. The sky was gray, thick, and hazy. My eyes were red, my throat was thick and sore and it hurt to breathe deeply. Even though there are more cars on the streets and freeways of LA now than 40 years ago, I haven't experienced that kind of aerial assault lately.

Treehugger's Brian Merchant, has had similar experiences:

My earliest memories of L.A. are colored with a grey, dystopian palette-I remember staring out at a hazed-over full moon, actually impressed by the way the smog smeared the city lights and hung thick in the air even at night. It was surreal and noirish and pretty repulsive. And that was just over ten years ago.

Like me, Merchant has been impressed to witness a typical day in LA with much lighter smog levels. In fact, the LA basin has seen vehicle-related air pollutants decrease by about 98 percent since the 1960s, even as locals are burning three times as much gasoline and diesel fuel since my day at the LA Zoo. That number comes from a National Oceanic and Atmospheric Administration report that tracks the trend, including findings on the air pollutants called volatile organic compounds that dropped by half between 2002 and 2010.

Cars are much cleaner and efficient than they used to be, but there are a lot more of them, which leads to three times as much fuel consumption as in the 1960s, the report says. Local residents and visitors do get stuck in traffic a lot, but the air is cleaner because technology has made breakthrough advancements. Some of those changes include requirements for catalytic converters, use of reformulated fuels less prone to evaporate and the improved engine efficiency of new vehicles.

All this does offer Angelenos, and others who live in congested mega-cities, a little bit of hope, even though smog is still present. According to the NOAA article, "People who lived in the city in the 1960s often couldn't see nearby mountains through the smog; today, they often can."