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-6246010/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

Figure 3-1.-Guidance Curve for Potential Fueling Hazards.

strength of the field presents a biological hazard to personnel located at the point where the measurements were taken. PEL readings are the basis for determining RADHAZ safety boundaries.

Permissible Exposure Time (PET)-This is the maximum time of exposure to a specific power density for which the PEL will not be exceeded when the exposure is averaged over any 6-minute period. Table 3-1 shows the PET for a variety of radars operated at their normal power levels.

If you suspect that you or someone else has been overexposed to EMR, follow the flow chart in figure 3-2. If you confirm your suspicions, the exposure is considered an incident and must be reported as required by Protection of DOD Personnel from Exposure to Radio Frequency Radiation, DOD Instruction 6055.11.

RFR HAZARDS TO THE SKIN.-The energy impinging on a person in an electromagnetic field may be scattered, transmitted, or absorbed. The energy absorbed into the body depends upon the dimensions of the body, the electrical properties of the tissues, and the wavelength of the RFR. Thus, the wavelength of the energy and its relationship to a person's dimensions are important factors bearing on the biological effects produced by RFR.

Significant energy absorption will occur only when a personal dimension is equivalent to at least one-tenth of a wavelength. As the frequency of radiation increases, the wavelength decreases and the person's height represents an increasingly greater number of electrical wavelengths, increasing the danger from RFR exposure. As the frequency is decreased, the wavelength increases and the person becomes a less significant object in the radiation field. Thus, the likelihood of biological damage increases with an increase in radiation frequency. Also, as the radiation frequency increases and the wavelength becomes progressively shorter, the dimensions of parts and appendages of the body become increasingly significant in terms of the number of equivalent electrical wavelengths.

When a person stands erect in a RFR field, the body is comparable to a broadband receiving antenna.

Figure 3-2.-Personnel RFR exposure decision chart.

Table 3-1.-Permissible Exposure Time Limits-Partial List

When any of the major body dimensions are parallel to the RFR energy's plane of polarization, the produced effects are likely to be more pronounced than when they are oriented in other positions.

The depth of penetration and coincident heating effects of energy on the human body depend on the energy's frequency. The region of transition between major damage and minor or no damage is between 1 and 3 GHz. Below 1 GHz, the RFR energy penetrates to the deep body tissues. Above 3 GHz, the heating effect occurs closer to the surface. At the higher frequencies, the body has an inherent warning system in the sensory elements located in the skin. At frequencies between 1 and 3 GHz, the thermal effects are subject to varying degrees of penetration, with the percentage of absorbed energy ranging from 20 to 100 percent. The two microwave cooking oven frequencies fall close to this range. The lower frequency, 915 MHz, produces a deeper heating effect on tissue (i.e., roasts) and is not as effective for surface cooking (browning) as the higher frequency, 2,450 MHz.

RFR HAZARD TO THE EYES.-The transparent lens of the eye may be damaged by radiated energy (ultraviolet, infrared, or radio frequency), causing the development of cataracts or opacities. The lens is very susceptible to thermal damage, since it has an inefficient vascular system to circulate blood and exchange heat to the surrounding tissues. Unlike other cells of the body, the cells of the lens cannot be replaced by regrowth. When cells in the lens die or become damaged, a cataract may form. The damaged cells may lose their transparency slowly and, depending upon the extent of damage, cause the individual to suffer impaired vision. Apparently, the presence of even a relatively few damaged cells may act upon other lens cells, either by releasing toxic substances or by preventing normal chemical transformation to take place within other cells.

RFR HAZARD TO THE TESTICLES.Testicular reaction to heat injury from excessive exposure to RFR radiation can be the same as the reaction to a high fever associated with many illnesses. Although a condition of temporary sterility may occur, the condition does not appear to be permanent and will ultimately correct itself. However, injury to the testicles may be permanent because of an extremely high dosage or because of high exposures for extended periods of time (i.e., months to years).

SHIPBOARD RADIATION HAZARD ZONES.-Because of the danger of radiation hazards to personnel, the fire control radar is equipped with cutout switches that turn off the transmitter for certain director bearings and elevations. The information concerning cutout zones for your particular installation is located in the radar OPs(Operational Publications). You should know the cutout zones for your particular radar. The equipment OPs also give the radiation pattern and the minimum safe distance for personnel exposed to the mainbeam of the radar. The safe limit of radiation exposure to personnel, established by the Naval Medical Command, is 10 mW/cm averaged over any one-tenth hour period (six minutes). No exposure in a field with a power density in excess of 100 mW/cm is permitted.