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Modern emission standards for cars in metropolitan areas have worked wonders in clearing out the smog that once blanketed cities everywhere. But like vaccines — which have worked so well in controlling disease that some misguided people don’t understand how necessary they still are — the dramatic effects of emissions standards won’t keep working if the standards are abandoned.

That’s why a Missouri proposal to exempt Jefferson and Franklin counties from automobile emissions tests goes in exactly the wrong direction. The Missouri Air Conservation Commission should reject it when members meet Thursday.

As the Post-Dispatch’s Jack Suntrup reported this week, the Missouri Department of Natural Resources is recommending that state air regulators remove the two counties from the St. Louis region’s vehicle emissions inspection program, which also encompasses St. Louis County and city and St. Charles County.

Those three jurisdictions would remain in the program — meaning drivers there will have to continue having their cars tested as part of the registration process to ensure their emission-control systems are working properly. Those systems prevent pollutants that elevate health-threatening ozone levels from being spewed into the local environment.

But under the department’s proposal, drivers in Jefferson and Franklin counties would no longer have to test their cars. The rationale for this proposed change is the height of irony: Those areas have so improved in air quality after years of those standards being in place that they’re no longer needed. Try applying that logic to, say, a working dam. Or, again, vaccines.

Proponents of the change point out that cars’ improving emissions-control systems are, by themselves, projected to continue reducing smog in the coming years. But those systems only accomplish that goal if they continue working properly — which is the whole point of emissions testing. Cars age; systems break down.

Most baffling is that the department acknowledges the projected improvement in air quality will be greater with continued testing. Consider how irrational this thinking goes: We could have cleaner air by just doing what we’re already doing — but let’s stop.

The emissions-testing requirement is, to be sure, an annoyance for drivers. So is stopping at stoplights. The convenience of operating automobiles comes with responsibilities, and they include emissions testing in urban areas to combat smog.

The Department of Natural Resources also is recommending eliminating the current requirement that St. Louis-area gasoline sales incorporate ethanol and other smog-reducing components. The move to drop those rules comes from the petroleum lobby, which tells you all you need to know about it.

The Air Conservation Commission meets Thursday to consider the proposals. Both should be rejected. The measurable success of emissions testing and reformulated gasoline in turning around the once-intractable problem of urban air pollution is cause to continue the course — not to abandon it.