What was hoped to halt the spread of the B.C. mountain pine beetle may actually be hazardous to human health.

B.C.'s Ministry of Forests is testing thousands of trees in the Okanagan-Shuswap region near Penticton to see if they contain a dangerous concentration of arsenic.

For two decades, ending in 2004, as many as 100,000 trees throughout the Interior were treated with monosodium methane arsenate (MSMA) in a bid to stop the spread of the mountain pine beetle.

Also known by its trade name, Glowon, MSMA's active ingredient is arsenic. Health Canada de-registered MSMA for use in 2005 when the manufacturer declined to have the product recertified.

The ministry is warning loggers and firewood cutters to avoid the treated trees.

The mountain pine beetle has devastated nearly half of B.C.'s marketable pine forest -- an area more than four times the size of Vancouver Island.

The infestation has been another hit to the province's beleaguered forestry industry, and swaths of trees have turned from green to a rusty red.

While scientists say its expansion through B.C. is slowing because it is running out of new trees to infest, the beetle is now pushing east past the Rocky Mountains and into Alberta.