Pathogen inactivation

A process designed to eliminate most pathogens — viruses, bacteria and fungi — from water, air or donated blood. Sewage purification systems depend upon pathogen inactivation to purify water to the extent it may sometimes even be safe enough to drink. Air purification systems, for example those incorporating a HEPA filter, also are designed to achieve pathogen inactivation and to cleanse the air of germs. With donated blood, the process of pathogen inactivation depends upon the fact that three components of blood that are given in transfusions — red blood cells to carry oxygen, platelets to help blood clot and plasma for clotting and other purposes — do not contain DNA or RNA, the basic genetic materials of life, whereas viruses, bacteria and fungi do. Therefore inactivating DNA or RNA can selectively kill these pathogens while leaving the blood itself unharmed. One form of pathogen inactivation for blood uses a chemical that, when exposed to ultraviolet light, binds to the genetic material. The bonds prevent the two strands of DNA's double helix from unzipping, thereby preventing germs from replicating. RNA, the genetic material in some viruses such as HIV, is similarly immobilized. There is current concern as to whether the technique can inactivate all viruses when they are present in large numbers. The technique is not designed to inactivate prions, which cause mad cow disease and its human equivalent, variant Creutzfeldt-Jakob disease, because prions are proteins and do not have DNA or RNA. And the process cannot be used to clean up white cell packs for transfusion because white cells have a nucleus and it contains DNA (so the process would inactivate the white cells). A platelet system uses a synthetic chemical known as a psoralen and a chain of three transparent plastic bags connected by tubes. The platelets are put into the first bag where they come in contact with psoralen. They drip into the second bag, which is placed in a machine photocopier to expose them to ultraviolet light for about three minutes. Then in the third bag, an absorbent material removes the psoralen. Different chemicals have to be used for the pathogen inactivation of red blood cells because light cannot penetrate these cells to activate a psoralen.