“ The term "duty involving frequent and regular exposure to highly toxic pesticides" shall be construed to mean duty performed by members who, while under competent orders assigning such members to the entomology, pest control, pest management, or preventive medicine functions of a uniformed service for a period of 30 consecutive days or more, are required to perform in any calendar month a fumigation task utilizing (1) phosphine, sulfuryl fluoride, hydrogen cyanide, methyl bromide, or (2) a fumigant of comparable high acute toxicity and hazard potential. ”