Physical hazards are hazards that result from energy and matter and the interrelationships between the two. Conceptually, physical hazards in the workplace can be subdivided into worker–material interfaces, the physical work environment, and energy and electromagnetic radiation. The consequences of exposure to these hazards can be modified by worker protection and a variety of human factors. This chapter will review the general principles of basic physics and worker protection.

Physics is the science of energy and matter and of the interrelationships between the two, grouped in traditional fields such as acoustics, optics, mechanics, thermodynamics, and electromagnetism. Quantum physics deals with very small energy forces; relativity deals with objects traveling at very high speeds (which causes time effects). Thus, physical hazards can be thought of as primarily hazards of energy, temperature, pressure, or time. This broad definition allows for the investigation of many hazards that are otherwise hard to classify but nevertheless represent important issues in the workplace. An understanding of these physical hazards requires familiarity with the two basic concepts of physics: classical mechanics, with its derivatives of thermodynamics and fluid dynamics, and electromagnetic radiation. For measurements, we have used Standard International (SI) units throughout this book, but we have included conversions to other units where they are in common usage. Table 1.1 reviews the standard unit prefixes for mathematics that are used in the physical hazards section. The mathematical equations for principles discussed in this section are included in tables that accompany the text. Although they are not necessary to understand the material, they are presented for those readers who wish to review them.

MECHANICS

Mechanics deals with the effects of forces on bodies or fluids at rest or in motion (Table 1.2). From mechanics, we can get to the study of sound, which is a result of the mechanical vibration of air molecules. The behavior of heat arises from the vibration of molecules. Temperature is proportional to the average random vibrational (in solids) or translational (in liquids and gases) kinetic energy. The physics of pressure arises from the laws of motion and temperature. The laws that govern electricity can be derived from special cases of mechanics (see below), and electromagnetic energy and waves are a direct result of the laws that govern electricity.

Classical mechanics is the foundation of all physics. Galileo (1564–1642) first described the study of kinematics. Kinematics is primarily concerned with uniform straight-line motion and motion where there is uniform acceleration. As a practical example, Galileo used these insights to predict the flight of projectiles. In uniform straight-line motion, velocity (v) is equal to the change in displacement (∆s) divided by the change in time (∆t). Acceleration (a) is the instantaneous change of velocity with respect to time, which is calculated by taking the derivative of velocity with respect to time. Where there is uniform acceleration, the new velocity is equal to the original velocity (v₀) plus acceleration times time. The distance traveled under acceleration is described by a combination of the component traveled at the original velocity plus the component traveled under acceleration. The mathematical equations for these forces are summarized in Table 1.3.

Sir Isaac Newton (1642–1727) originally described the study of mechanics in his 1687 Philosophiæ Naturalis Principia Mathematica. He formulated three laws that serve as the foundation of classical mechanics (Table 1.4).

The first law is known as the law of inertia. It states that all matter resists being accelerated and will continue to resist until it is acted upon by an outside force. The second law states that the acceleration of this outside force will be related to the size of that net force (F) but inversely related to the mass (m) of the object. This relationship is described mathematically by the following expression:

The third law states that when two bodies exert a force on each other, they do so with an action and reaction pair. The force between two bodies is always an interaction.

A good example of how all three laws operate can be seen at the bowling alley. When a bowling ball is sitting on the rack, the force of the ball pressing down on the rack (gravity) is equal and opposite to the force of the rack pressing up on the ball to resist gravity (the third law). The speed of the bowling ball at the end of the alley is dependent on the amount of acceleration imparted to it. An adult can apply more force to the ball than a child, so the adult’s ball will go faster. However, if smaller balls (i.e., of less mass) are used, less force is required; therefore, a child can accelerate the ball to the same speed (the second law). Once the ball leaves your hand, no more net force is applied to the ball (if we ignore friction), and it travels down the alley at a constant speed (the first law).

Mechanics has been central to the advancement of physics. Two mechanical concepts are central to understanding what strategies to adopt in order to prevent injury and illness from physical hazards: kinetic energy and potential energy. In order for physical hazards to affect humans, they must possess energy to impart to the biological system. Energy is commonly described in terms of either force (F) or work (W). Force equals mass times the acceleration, and the result is a vector. F = ma is the mathematical representation of Newton’s second law. The work done on an object equals the amount of displacement times the force component acting along that displacement. In the special case of the force acting parallel to the displacement, work equals force times displacement. These two relationships are described mathematically in Table 1.5, equations 1 and 2.

Kinetic energy (KE) is the energy of a mass that is in motion relative to some fixed (inertial) frame. KE is related to the mass of the object, and the speed at which it is traveling (Table 1.5, equation 3). Potential energy (PE) is stored energy that can do work when it is released as kinetic energy.

Since mass and energy are conserved in all interactions, the sums of potential and kinetic energy from before and after an encounter are equal. The equation for kinetic energy is also important for electromagnetic radiation. An electric system can store electric energy in a magnetic field in an induction coil. The kinetic energy of the electric charges equals the amount of work done to set up the field in the coil, which is stored as potential energy.

Work, kinetic energy and potential energy in this system are related to the inductance of the coil and the current (Table 1.5, equation 4). Potential energy is the potential to do work, and theoretically all this work can be turned into kinetic energy. The expressions for kinetic energy in the mechanical system and potential energy in the electric system have an identical form. This form shows the similarity between kinetic and potential energy in mechanics and electromagnetic radiation and lays the groundwork for examining the electromagnetic wave.

ELECTROMAGNETIC RADIATION

By far the most complicated concept related to the understanding of physical hazards is that of electromagnetic radiation (EMR). Energy can be transmitted directly by collision between two objects, or it can be transmitted by EMR. We see direct examples of energy transfer by EMR when we are warmed by the infrared rays of the sun or burned by its ultraviolet rays. EMR is a continuum of energies with different wavelengths and frequencies. Two similarities of all types of EMR are that they all move at the same speed and they are all produced by the acceleration or deceleration of electric charge. EMR has a dual, particle–wave nature: its energy transfer is best described by a particle, but the behavior of the radiation is best described as a wave. All EMR travels at a constant speed, c = 3 × 10⁸ m/s (the speed of light). Each particle of energy, called a photon, is accompanied by an electric field (E-field) and a magnetic field (H-field); these fields are perpendicular to each other and perpendicular to the direction of travel of the wave (Figure 1.1).

It is important to remember that EMR is only produced when an electric charge is moving. Coulomb forces are forces between stationary charges, whereas magnetic forces are due to the motion of charges relative to each other. A moving electric charge (or electric field) induces a magnetic field, and a moving or changing magnetic field induces an electric field. In 1873, James Maxwell linked together these electric and magnetic phenomena into a unified field theory of EMR. As an electric charge moves, it induces a magnetic field, which in turn induces an electric field. The mutual interaction of these two fields is what allows the electromagnetic wave to propagate and what dictates its physical form in Figure 1.1.

The energy (E) in each photon in the wave can be calculated in joules (J) and is related to the frequency of the radiation in hertz (Hz). Energy is calculated by multiplying the frequency by Planck’s constant (6.626 × 10^–34 Js). The mathematical representation of this is shown in Table 1.6, equation 1.

Since the velocity at which the wave travels equals the frequency times the wavelength (Table 1.6, equation 2), we can discover the wavelength (λ) for each frequency by dividing 3 × 10⁸ m/s (the speed of light or c) by the frequency (Table 1.6, equation 3). The energy of the wave can also be calculated in terms of the wavelength by substituting the speed of light divided by the wavelength for frequency (Table 1.6, equation 4). In biological systems, it is useful to determine photon energy in electron volts from the wavelength. This can be calculated from the wavelength in angstroms (Å) according to Table 1.6, equation 5. The electron volt is a convenient unit to use with biological systems, because it takes greater than roughly 10  electron volts (eV) to cause ionization in tissue.

We can also see from equations 1 and 2 in Table 1.6 that the energy of a given type of EMR varies directly with its frequency and inversely with its wavelength. Figure 1.2 shows a representative cross section of the electromagnetic spectrum, with the major classes noted. Notice that there are not strict divisions between the different classes of EMR. An important division in the EMR spectrum relates to the ability to ionize chemical bonds in biological tissue. As frequency increases from the radio bands, so does energy, until ionization potential is reached in the “hard” ultraviolet or “soft” X-ray bands.

A final important point about EMR involves the ways in which it can interact with objects. EMR interacts with biological tissues in one of the following three ways: (i) transmission, where the radiation passes through the tissue without any interaction; (ii) reflection, where the radiation is unable to pass through the air–tissue interface (also called the boundary layer) and is reflected back into space; and (iii) absorption, where the radiation is able to pass through the boundary layer and deposit its energy in the tissue. The frequency of the EMR determines what energy is released in the tissues (heat, electric potential, bond breaking, etc.). These interactions are summarized in Figure 1.3.

WORKER PROTECTION

Potential energy can also be called a potential hazard. The key to avoiding injuries and illnesses is to prevent the individuals in the workplace from being overexposed to the kinetic energy in the hazards. The major characteristics of the physical hazards covered in this text are reviewed in Table 1.7. Each of the following chapters will deal with the most appropriate method to prevent overexposure. However, there are certain recurring themes.

Since we are trying to prevent exposure, the first step is to educate the workforce. A good training program includes education about the potential hazards, the safest procedures to follow for each manufacturing or maintenance operation, correct tool selection and use for each job, use and care of personal protective equipment, and procedures to follow in emergency situations, including fire and loss control, shutdown, rescue, and evacuation.

Substitution of less dangerous equipment or agents is the best protection from hazards, because it totally removes any chance of exposure. However, substitution is often not possible; therefore, worker protection from physical hazards generally focuses on engineering controls. Engineering and administrative controls for physical hazards are summarized in Table 1.8. Often, these controls involve isolation or shielding from the hazard. The most effective isolation involves physically restricting an individual from a hazard area by fencing off the area whenever the hazard is present. Interlocks that inactivate the equipment when the exclusion area is entered are often used to further enhance physical barriers. Alternatively, the hazard can be “locked out” when a worker is present in an area that would become hazardous if the equipment were energized (Chapter 5). This process of excluding maintenance workers from hazardous areas has been institutionalized in the Occupational Safety and Health Administration (OSHA) lockout/tagout (LOTO) standard (Code of Federal Regulations [CFR] 1910.147).

Another way to protect workers is to specifically shield them from the hazard. In some cases, an individual piece of equipment can be shielded to prevent exposure. With some higher-energy hazards, such as ionizing radiation, shielding may be needed in addition to isolation of the equipment. In special cases where it is not practical to shield the hazard (e.g., cold, low pressure), individual workers can be shielded with personal protective equipment, such as jackets or environment suits. In addition, it is sometimes possible to alter the process so as to decrease exposure. This is often the case with hazards affecting the worker–material interface, where engineering design is often inadequate. Personal protective equipment can also be used as an adjunct to engineering controls. Table 1.9 contains a summary of the most common personal protective equipment used for physical hazards.

The final strategy for hazard control is the use of administrative controls. These controls are implemented when exposures cannot be controlled to acceptable levels with substitution, engineering controls, or personal protective equipment. Administrative measures can be instituted to either rotate workers through different jobs to prevent repetitive motion injuries or to remove workers from ionizing radiation exposure once a predetermined exposure level is reached. Although this is not the preferred method of hazard control, it can be effective in some circumstances. Administrative controls are also reviewed in Table 1.8.

The best way to determine what hazards are present in a specific workplace is to go to the site and walk through the manufacturing or service process. There are a number of excellent texts available on evaluating workplaces from both an industrial hygiene and a safety perspective; they are included in the list of further reading at the end of this chapter. An additional point that will become obvious as you read through the text is that there are some significant measurement issues that need to be addressed by an appropriate health professional. Although larger employers will undoubtedly have such a person on staff, at the majority of smaller work sites, no such person will be available.

If you are unfamiliar with the measurement technology, make sure that you (or the employer) retain someone who knows how to perform an exposure assessment. Inaccurate measurements will invalidate the entire process of a prevention program. There are, of course, a number of physical hazards that do not require special measurements and can be handled quite nicely with relatively low-cost safety programs. Several excellent texts describing how to set up general safety programs are included in the further reading list at the end of this chapter.

Finally, remember that the human being is a biological system. For a given exposure, different people will respond differently because of interindividual variation. Most workplace standards are designed with a safety factor to protect against overexposure related to this variation (and to account for any knowledge gaps). In addition, a worker’s perception of the hazard must also be taken into account. Some workers may have an exaggerated response to a nonexistent or low-threat hazard, whereas others may not respond appropriately to a series hazard with which they have “grown comfortable.” The challenge in assessing and communicating the relative danger entailed by the hazard is to strike the right balance between these two competing tendencies.

The goal of the first section of this volume is to acquaint the reader with the types of physical hazards that may be present in the workplace. Once these hazards are identified at the site, he or she can refer to the specific chapter that addresses the salient measurement issues or offers general strategies to control exposures and monitor effects.

Further Reading

1. Balge MZ, Krieger GR. Occupational health and safety, 3rd edn. Chicago: National Safety Council Press, 2000.
2. Burgess WA. Recognition of health hazards in industry: a review of materials processes, 2nd edn. New York: John Wiley & Sons, Inc., 1995.
3. Hagan PE, Montgomery JF, O’Reilly JT. Accident prevention manual for business and industry: administration and programs, 14th edn. Chicago: National Safety Council Press, 2015.
4. Plog B, Quinlan P (eds.). Fundamentals of industrial hygiene, 6th edn. Chicago: National Safety Council Press, 2012.
5. Serway RA, Vuille C. College physics, 9th edn. Boston: Cengage Learning, 2011.
6. Spitz H, Albert RE. Ionizing Radiation. In Bingham E, Cohrssen B (eds.) Patty’s toxicology, 6th edn. New York: John Wiley & Sons, Inc., 2012, pp. 1–23.