Ammunition makers switched to a "non-toxic" alternative to lead but the replacements may not be better, according to a new study.

The federal government began phasing out the use of lead shot in 1999 to cut down on the amount of the pollutant, which can poison birds.

Lead was replaced with bismuth, which behaves ballistically in a similar way and is cheaper than steel shot.

A federal regulation requires non-toxic shot to be used on migratory birds.

Despite studies supporting the change, scientists did not know what the background levels of bismuth were in the environment.

Researchers at the University of Toronto at Scarborough, Queen's University in Kingston, Hamilton's McMaster University, and the University of Waterloo reviewed the levels of lead and bismuth from waterfowl harvested with lead shotshell.

Biochemistry Prof. Evert Nieboer of McMaster and his colleagues looked at background levels of the two pollutants in muscle and liver tissues from:

• Mallard ducks.
• Northern pintails.
• Green-winged teals.
• Canada geese.
• Snow geese.

Hunters from Cree communities in the western James Bay region, who eat the birds as part of their traditional diets, provided the samples.

The researchers found signs of lead contamination from the bismuth shot. Excessive exposure to bismuth has been linked to blood, liver, kidney and neural problems.

The levels detected suggest there were analytical errors in previous studies on the effects of bismuth in birds, the researchers said.

Since bismuth is a contaminant of lead, the researchers said more research is needed to confirm if bismuth is an appropriate "non-toxic" alternative to lead shot.

The study appears in the November issue of the journal Environmental Pollution.