Duties of 25 percent on steel and aluminum will flow through to car buyers, beer drinkers, home builders, oil drillers and other users of metal goods.

European leaders have said that they will respond firmly to higher levies on steel and aluminum, but they have yet to say what that retaliation will look like.

Existing American tariffs already restrict steel and aluminum shipments from China, which is now flooding other markets with its exports instead.

R.F.K. Jr. and others have blamed the ingredient for allergies and other illnesses. Scientists say it actually bolsters the immune response.