The EPA's Office of the Inspector General, the internal agency watchdog, said this week that improvements to the agency's emissions testing since the Volkswagen diesel scandal have been effective, but that more could be done.

According to a report in The Hill, emissions testing improvements have been effective overall since Volkswagen duped regulators regarding the emissions compliance of its TDI models. 

The diesel cheating was uncovered by researchers at West Virginia University working on behalf of the International Council on Clean Transportation, which was preparing a report on the success of clean diesels. After the testing concluded that the diesels emitted up to 35 times the allowable emissions of smog-forming oxides of nitrogen, the report was never published, and the ICCT reported the findings to the EPA instead.

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In addition to diesel cheating, the scandal brought to light several inadequacies of the EPA's antiquated emissions testing regimen in an age of electronically controlled cars.

The EPA emissions tests were originally devised in 1975, using drive cycles that reflected traffic patterns and technology of the day. New test cycles were added in 2008 to reflect more modern driving with higher-powered cars and near-universal installation of air conditioning.

Automakers conduct most of the testing themselves and report data to the EPA. The EPA spot-checks some vehicles to ensure accuracy.

READ THIS: EPA Will Add On-Road Testing For Emissions Compliance (2015)

The diesel scandal brought to light technological cheating that allowed cars to detect test cycles and bring emissions into compliance during only tests and to pollute more in normal driving. Since then, the EPA has increased spot checks and has begun using more modern equipment that tests emissions in real-world driving, sometimes by mounting test equipment to the back of a car while it's on the road.

The Inspector General's report [PDF] says these measures have been successful in identifying and deterring emissions cheating, but recommends that the new measures could be even more effective if they were better integrated into the Agency's regular test procedures.

EDITOR'S NOTE: An earlier version of this article stated that the Volkswagen diesels emitted up to 37 times the legal limit of nitrogen oxides. According to the original announcement of the study posted by West Virginia University, the actual number was 35.