Hazards of Electromagnetic Radiation to Fuels (HERF) Many  studies  have  been  done  about  fuel  vapors being   accidentally   ignited   by   electromagnetic radiation. Tests aboard ships and in laboratories have shown   that   the   chances   of   this   happening   are   low because of other conditions that must exist at the same time   to   support   combustion   of   the   fuel.   Although accidental ignition of fuel by RFR is unlikely, you still need  to  be  aware  of  the  potential  hazards.  The  most likely time this might occur is during a ship’s refueling evolutions,   commonly   called   UNREPs   (Underway Replenishment).  Many  ships  also  carry  at  least  one helicopter or have the ability to refuel a helicopter and, therefore, carry fuel to support helo operations. All of these   operations   are   inherently   dangerous   by themselves   and   require   the   utmost   attention   and alertness. As a junior Fire Controlman you most likely will   be   personally   involved   in   these   refueling operations.   You   need   to   be   aware   of   the   potential hazards  associated  with  Fire-Control  radar  and  fuel. As  a  senior  Fire  Controlman,  you  need  to  know  the hazards  of  electromagnetic  radiation  to  fuel,  so  you can ensure that your division personnel are working in a safe environment. RADAR   RESTRICTIONS.—Electromagnetic Radiation  Hazards  (U)  (Hazards  to  Personnel,  Fuel and  Other  Flammable  Material)  (U),   NAVSEA  OP 3565/NAVAIR16-1-529/NAVELEX   0967-LP-624- 6010/Volume   I   specifies   the   safe   distances   from radiating sources at which fueling operations may be conducted. Figure 3-1 indicates safe distances between fueling  operations  and  a  conical  monopole  antenna, based on transmitter power. Each type of antenna has its own chart. Refer to your ship’s Emissions Control (EMCON)   bill   for   specific   guidance   concerning fueling operations. FUEL  RESTRICTIONS.—As  the  RFR  energy radiated   from   high-powered   communications   and radar equipment installed on ships increased in recent years,  the  Navy  shifted  to  less  volatile  fuels.  Under normal   operating   conditions,   volatile   mixtures   are present only near aircraft fuel vents, open fuel inlets during over-the-wing fueling, and near fuel spills. Before   fuel   vapors   can   ignite,   three   conditions must exist simultaneously: 1.   For  a  given  ambient  temperature,  the  mixture must contain a specific ratio of fuel vapor to air. 2.   There must be enough energy in the arc or spark to   produce   the   appropriate   temperature   for ignition. 3.   The   length   of   the   arc   must   be   sufficient   to sustain the heat in the arc for the time required to initiate a flame. Each of these conditions is likely to vary for every situation, and two of the conditions may exist at any given time. Although all three conditions will probably not   occur   simultaneously,   the   consequences   of   an accidental   explosion   make   it   very   important   to   be careful. Hazards of Electromagnetic Radiation to Personnel (HERP) The   RFR   hazard   category   of   most   immediate concern to you is HERP. The heat produced by RFR may adversely affect live tissue. If the affected tissue cannot   dissipate   this   heat   energy   as   fast   as   it   is produced,  the  internal  temperature  of  the  body  will rise. This may result in damage to the tissue and, if the temperature rise is sufficiently high, in death. The   Bureau   of   Medicine   and   Surgery   has established  safe  exposure  limits  for  personnel  who must  work  in  an  electromagnetic  field  based  on  the power  density  of  the  radiation  beam  and  the  time  of exposure in the radiation field. Before we discuss these further, we must discuss some additional terms. Specific Absorption Rate (SAR)—This is the rate at   which   the   body   absorbs   non-ionizing   RFR.   The threshold at which adverse biological effects begin is 4 watts per kilogram of body mass (W/kg). With a safety factor of 10 added, the accepted threshold is 0.4 W/kg for the whole body, averaged over any 6-minute (0.1 hour) period. A special limit for “hot spot” or limited body exposure has been set at 8.0 W/kg, averaged over any  1  gram  of  body  tissue  for  any  6-minute  period. Although this rate of absorption is very important in determining whether or not a safety hazard exists, it is very   difficult   to   measure.   Measuring   this   rate   of absorption   can   also   be   dangerous   since   it   requires actual exposure of body tissue. A related measure that gives an acceptable indication of SAR is “Permissible Exposure Limit”. Permissible  Exposure  Limit  (PEL)—This  is  a limit  to  RFR  exposure  based  on  measurements  of radiation’s  electric  field  strength  (E)  or  magnetic field  strength  (H)  taken  with  instruments.  You  can use   available   charts   to   determine   whether   the 3-3