Curtis Johnson CURT JOHNSON: As the region most threatened by pollution in the entire nation, California’s Central Valley, stretching from Modesto southward to Bakersfield, is center stage in the struggle to sustain breathable air.

There recently to speak at a conference produced by the San Joaquin Valley Air Pollution Control District, I might have found it memorable for a surprise encounter with a minor earthquake. But the truly unsettling experience was the encounter with the Valley’s air quality dilemma. I was there on a “clear” day, but it wasn’t something Robert Goulet would break into song over. You surely couldn’t see “forever,” not even to the Sierra mountains. But locals said this is as good as it gets.

Think of the valley like a giant 28,000 square mile bathtub, with mountains for sidewalls. The winds prevail north to south until they hit the southerly range, where air stops and stands still. The mountains not only contain air but overspread a cooler layer that acts like a lid covering a toxic stew of particulates and nitrogen oxides, which then bake and form even more noxious gases. Most of these are unseen microparticles but are the reason one in six kids in the Fresno schools has asthma symptoms.

Adding to geography is a usually hot climate, just right for baking gases. Then add a high rate of growth, despite chronic double-digit unemployment rates, compounded by the mix of the valley’s major industries — agriculture and (in south valley) oil and gas — and you have an environmental challenge like no other place in America.

The good news: first, this summer was cooler than usual, so there were more clear days. And, over the last generation cars and trucks have improved so much that their toxic emissions are down 90 percent. This matters, since well over half of all the pollution comes from vehicle traffic, including the 20,000 large trucks that traverse the valley every day.

More good news: both oil and gas and the agricultural industries have made giant gains in reducing source-point pollution. Better emission controls and processing techniques, less wasteful drip irrigation methods, and conversation-tillage processes combine to reduce both nitrous and particulate poisons. Without these gains, there’d be no hope for improvement. South Valley is home to the fourth largest source of oil and gas in the nation. And the Valley overall produces something like 28 percent of the fruits and vegetables consumed worldwide.

The best news is the emergence of regional organizations, backed by the biggest businesses in the valley. One is Operation Clean Air. Another is Valley CAN (for Clean Air Now), with a board stocked with representatives from the food, diesel engine, agricultural, and real estate industries. They’ve taken direct aim at reducing diesel, fireplace, and older vehicle emissions, along with reducing dust.

As winter nears, and we all assume that steady supply of fruits and vegetables at the local grocery store, just remember that the source of much of it is locked in a struggle to keep all that good food moving our way.