The biggest natural foods markets, Whole Foods and Wild Oats , have the widest selection of organic food that you can find anywhere. But both chains also offer some so-called conventional produce.

These chains aren’t organic purists. For example, last year I argued unsuccessfully with the fish buyer for Wild Oats that it is contrary to that company’s philosophy for it to be selling farmed salmon, which is generally contaminated with dangerous levels of polychlorinated biphenyls (PCBs). But since wild salmon is available for only a few months of the year, Wild Oats – as well as Whole Foods and practically everyone else – still sells farmed salmon. At least Wild Oats identifies it as such.

Conventional produce at both major natural foods markets is almost always less expensive at the checkout stand. But their external or hidden costs – including taste, health, pollution, government subsidies – are huge, as Michael Pollan carefully analyses in his outstanding new book, The Omnivore’s Dilemma. My previous post pointed out why people with diabetes especially need to eat organic produce.

People from both of these natural foods markets tell me that they offer conventional produce as a “convenience to the shopper.” What that means is they want to discourage their customers from shopping in the Kroger’s and Safeway’s that still sell most of the food in America.

Buying conventional produce at Whole Foods and Wild Oats may be convenient in the short run. But conventional fruit and vegetables are certainly contaminated with high levels of pesticides.

The problem is that nobody knows how much of this pesticide is too much for humans to consume. So it makes sense to eat as little pesticide as we can.

At a minimum this means avoiding those foods with the most pesticide on them. But even Whole Foods and Wild Oats sell – as a so-called convenience – some fresh fruits and vegetables that are consistently the most contaminated with pesticides.

At least Wild Oats now tells me which of these fruits and vegetables to avoid. A few days ago the company’s weekly email told me about the Environmental Working Group’s (http://www.ewg.org/ ) “dirty dozen” foods most contaminated with pesticides.

Typically, the dirty dozen have from two to three different pesticides in each sample. They found up to 10 different pesticides is a single sample and up to 45 different pesticides in one kind of fruit.

Eight of the 12 most contaminated foods are fruits. The worst? Peaches.

I would have guessed that strawberries are the worst, but they are only number 2. Apples, nectarines, pears, cherries, red raspberries, and imported grapes follow.

What about vegetables? Four of the dirty dozen are vegetables.

My favorite lunch is a vegetable salad, and one of my favorite salad bars is Sweet Tomatoes. With about 100 restaurants in 15 states – called Souplantation in Southern California and Sweet Tomatoes elsewhere – this chain offers the most vegetable ingredients for my salad. An imitator, Fresh Choice, has 29 restaurants in three states.