Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

2.4 mW
cm 2
E mW
cm 2
= (50 cm)2
(10 cm) 2
E = 0.096 mW/cm 2 .
(b) With Eq. (14.2), we find
UV Protection
J
0.003
t =
cm2 = 31.25 seconds.
W J/s
0.096 × 10−3 × 1
cm2 W
NONIONIZING RADIATION SAFETY 727
If a longer exposure time is required, then the laboratory worker must use a
barrier cream or personal protective equipment, such as goggles and gloves
that will absorb the UV radiation.
The basic principles for safe use of UV radiation are the same as for ionizing radiation:
time, distance, and shielding. An additional safety consideration when working
with intense UV sources is that short-wavelength UV can produce ozone (O3) in measurable
quantities. Ozone is a pulmonary irritant, and has an Occupational Safety
and Health Administration (OSHA) permissible exposure limit (PEL) of 0.1 ppm.
Shielding, on a personal level, can be accomplished through the use of eyeglasses,
goggles, or plastic face shields to protect the eyes and face. These items must, of
course, be effective against the UV wavelengths in use. Ordinary glass blocks most
UV light of wavelengths less than 330 nm, while transmitting most of the UV for
longer wavelengths. Ordinary safety glasses and goggles use polycarbonate lenses,
which absorb 100% of the UV. The UV-absorption properties of contact lenses,
ordinary eyeglass lenses, sun glasses, and protective clothing can be determined by
measuring the transmission of UV light with a radiometer.
Food and Drug Administration (FDA) regulations for sunscreen creams and lotions
are published in 21 Code of Federal Regulations (CFR) 352. These blocking
agents are designed only for UVB and UVA. The ability to block UVB is measured by
the sun protection factor, SPF. The SPF is defined as the fraction of the incident UV
that is transmitted by a thickness of 2 mg/cm 2 . Thus, an SPF of 20 means that 1/20,
or 5% of the incident radiation is transmitted, and 95% is absorbed. The percent of
the incident UV that is absorbed is given by
% absorbed =
SPF − 1
SPF
W Example 14.3
× 100. (14.3)
Calculate the percentage of UVB absorbed by a sunscreen with an SPF = 30.