calls to fire stations, utilities and emergency response teams when the levels of co are not life threatening. this change will also reduce the number of calls to these agencies due to detector inaccuracy or the presence of other gases. consequently, new alarms will not sound at co concentrations up to 70 ppm. note that these concentrations are significantly in excess of the canadian health guidelines," (and also in excess of us occupational safety and health administration (osha) permissible exposure limits, which is 50 ppm). uk in the uk a domestic/type-b alarm compliant with bs en 50291:2001 should emit an audible alarm after about 3 minutes exposure to 300 ppm co, or 10 to 40 minutes at 100 ppm, or 60 to 90 minutes at 50 ppm.