2005 Senate confirmation hearing, EPA was criticized for support of using human subjects in pesticide testing. Johnson "did not have the opportunity to fully address the committee's criticisms before the hearing was recessed." In April 2005, a secret hold was placed on his confirmation vote while he evaluated the Children's Environmental Exposure Research Study, which advocated recording the effects of pesticides on children from infancy to age 3. In a letter that reached Senator Barbara Boxer several hours after she raised her concerns, Johnson said, "No additional work will be conducted on this study subject to the outcome of external scientific and ethical review." On April 8, Johnson canceled the study. His nomination was confirmed by the United States Senate on April 29. On February 6, 2006, he issued a final regulation "prohibiting new research involving intentional exposure of pregnant women or children intended for submission to the EPA under the pesticide laws" and other protections. Johnson tried to block the efforts of 17 states to reduce greenhouse gas emissions and improve fuel economy. He defended his position by arguing that "The Bush administration is moving forward with a clear national solution, not a confusing patchwork of state rules. I believe this is a better approach than if individual states were to act alone." The state rules he was blocking were more stringent than the Bush administration's proposed national solution. Johnson came under investigation for allowing the White House to improperly interfere with the decision to grant California a waiver to limit greenhouse gases. On May 20, 2008, Johnson was questioned for three hours by the House Oversight and Government Reform Committee. On July 29, 2008, four Senators called for Johnson's resignation, alleging he made false statements to Congress. On December 9, 2008, the Office of Inspector General, US EPA concluded that "EPA's California waiver decision on greenhouse gas automobile emissions met statutory procedural requirements." On May 19, 2009, President Obama also concluded: "a clear and uniform national policy is also good news for the auto industry which will no longer be subjected to a costly patchwork of differing rules and regulations." Johnson's stance on this and other issues was criticized in an editorial by the scientific journal Nature, which claimed he acted with "reckless disregard for law, science or the agency's own rules — or, it seems, the anguished protests of his own subordinates." In spite of this external