He said in his opinion, insurgents are probably "experimenting with different ways of releasing harmful materials as an indirect effect of explosions."

Hospital officials have been reporting more serious burn victims, both among the dead and wounded, in recent attacks.

About an hour before the market was hit, a suicide car bomber crashed into an Iraqi police checkpoint at an entrance to Sadr City, the capital's biggest Shiite Muslim neighborhood and a stronghold for the Mahdi Army militia.

The explosion killed at least 41 people, including five Iraqi security officers, and wounded 76, police and hospital officials said.

A towering column of black smoke rose from a tangle of eight incinerated vehicles that were in a jam of cars stopped at the checkpoint. Bystanders scrambled over twisted metal to drag victims from the smoldering wreckage. Iraqi guards who survived the bombing staggered through the carnage, apparently stunned.

During the noon hour, a parked car exploded near a private hospital in Karradah, a predominantly Shiite district in the center of Baghdad. At least 11 people died and 13 were wounded, police said. The blast damaged the Abdul-Majid hospital and other nearby buildings.

The fourth bombing exploded in a small bus in the central Rusafi area, killing four people and wounding six, police said.

In other violence, a suicide bomber struck a police patrol at nightfall in the Saydiyah neighborhood, a mixed Sunni-Shiite district in southwest Baghdad. Four died, including two policemen, and eight were wounded, five of them police, police officials said.

The U.S. military also said a suspected insurgent was killed and eight captured in two raids north of Baghdad on Wednesday. Some of the suspects were believed linked to al-Qaida in Iraq and to a militant cell that has used chlorine in truck bombings, the statement said.