Every time there's bridge-painting, they wrap the bridges to prevent debris from falling into the river. But on Cinco de Mayo, the Fourth of July, etc., the river is used as an ashtray for the heavy metals and toxic chemicals in fireworks. Isn't there a smarter way to spend our tax dollars?

—Ray

When I told one of my housemates what I was working on this week, she asked, brightly, "Are you going to tell them that the solution to pollution is dilution?"

"No," I said, and made her wash the dishes. That said, when you're deciding whether to be worried about an environmental chemical, the question "How much?" is at least as important as the question "What kind?"

Let's take cadmium, the most dangerous colorant in fireworks. Cadmium will mess up your lungs, kidneys and bones and give you cancer. The Environmental Protection Agency says that levels of cadmium above 5 parts per billion are unsafe.

That's not much—by comparison, 4 parts per billion is the ratio of winning Powerball tickets to all Powerball tickets. However, it's also true that cadmium occurs naturally in the earth's crust—both seawater and soil contain about 0.1 parts per billion of it all the time. So "how much" matters.

The Willamette's flow rate means about 3.7 billion pounds of water pass during a half-hour fireworks display. Even assuming the manufacturers used an entire pound of cadmium powder/foil per show, the river's contamination wouldn't reach 0.4 parts per billion.

The Bureau of Environmental Services says it doesn't test for fireworks residue. Among places that have, though, the only worrisome concentrations I could find were at a landlocked lake in a tourist town that had fireworks shows every week.

Meanwhile, Multnomah County reports that the recent partial repainting of the Broadway Bridge shipped over 100,000 pounds of debris. That's a lot. Given that the only thing Portland loves more than a fireworks show is yet another bridge closure, it's probably just as well that jimmy wears a hat.

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