Use and Handling of Hazardous Chemicals
In this part, we will consider the hazards, precautions, and emergency procedures pertinent to the safe use and handling of chemicals. There are two categories to bear in mind: chemical hazards and physical hazards. But first, we need to briefly discuss labels and MSDSs.
Labels and Material Safety Data Sheets
Precautionary labels for chemicals typically present information in four parts, usually in the order described here. First is a Signal Word: “Danger”, “Warning”, or “Caution”. Only one of the three should be used on a label. “Danger” is the strongest of the three and is used when the contents present a potential for serious foreseeable harm. “Caution” is restricted to chemicals that are foreseeably the least potentially harmful. “Warning” is for chemicals intermediate in their potential to cause foreseeable harm. One or more Statements of Hazard follow the Signal Word.
These are succinct descriptions of the major foreseeable way or ways in which the chemical could cause harm. Examples include “Flammable”, “Harmful if Inhaled”, “Causes Severe Burns”, “Poison” (with or without a skull and crossbones graphic), and “May Cause Irritation”. Chemicals that exhibit two or more hazards are labelled with a corresponding number of Statements of Hazard. Next on the label are one or more Precautionary Measures, as appropriate. These are brief descriptions of actions to be undertaken or avoided and which, if heeded, will prevent the corresponding hazard(s) that are described by the Statements of Hazard from causing harm. Examples include “Keep Away from Heat, Sparks, and Flame”, “Use with Adequate Ventilation”, “Do Not Get in Eyes”, and “Avoid Breathing Dust”. Usually, but not always, First Aid or other information will appear on a label below, or off to the side of, the Precautionary Measures.
Typical First Aid information includes instructions such as how to induce vomiting or to not induce vomiting if that is the case. Advice to wash off the skin or flush the eyes if the victim has been exposed to a corrosive chemical, how to extinguish a fire involving a chemical, and what to do if an excessive amount has been inhaled are also included. Typically, MSDSs contain similar information, but in more detail, and frequently in a different order from that used for labels. Usually, MSDSs are not written for the layperson; they require interpretation by persons familiar with the technical terms used. Often, an overemphasis is placed on the toxic characteristics of the subject chemical.
There may also be vague or insufficient information regarding other hazards that the subject chemical presents. MSDSs vary widely in quality and reliability. Generally speaking, MSDSs from well known, established suppliers of laboratory chemicals are likely to be better and more reliable than MSDSs from other sources. Often, a comparison of MSDSs for the same chemical from a variety of suppliers will suggest a source of MSDSs that is likely to be the most authoritative. Chemists and others who have an interest in the local school community can be resource persons to help supervisors and teachers understand the language and evaluate the content of an MSDS. The use of these resource persons is strongly encouraged.
The hazards presented by any chemical depend upon the properties of that chemical. Each chemical is different from all others because it has properties that are different. So, it follows that each chemical presents different hazards.
But to use a chemical properly, first we must know the hazards of that chemical; second, we must know and apply the appropriate precautionary measures that will reduce the probability of harm from those hazards; and third, we must know and be prepared to carry out the necessary emergency measures (should our precautions fail) that will minimize the harm, just in case. It would seem that these requirements are formidable. How can I know that much about each of the many chemicals my students and I will use in the lab— to say nothing of teaching all this to the students? Fortunately, there is a practical answer: classification.
Chemicals present only four classes of chemical hazards: • Flammability • Corrosivity • Toxicity • Reactivity The following sections describe each of these hazards separately. Keep in mind that any single chemical may simultaneously present more than one hazard. A few chemicals also possess physical hazards, which are discussed later. But before attending to these hazards, there is one all-important precautionary measure that requires the first-place mention in any discussion of chemical hazards: eye protection.
Always, when hazardous chemicals are used or handled, when glassware is used or handled, when flames are involved, all persons present, whether or not they are doing the handling or using, must wear eye protection. Ordinary spectacles do not provide protection from chemical splashes; even spectacles with so-called hardened lenses do not provide this kind of protection.
Similarly, contact lenses alone are not considered to offer sufficient protection when used without safety goggles. Only safety goggles (also known as chemical splash goggles) as described below and marked with the code “Z87” provide the kind of protection that is needed. The Z87 code refers to a voluntary standard promulgated by the American National Standards Institute called ANSI Z87. This standard describes several different kinds of eye and face protection, all of which can be purchased from suppliers and bear the Z87 code marking. For example, a type of eye protection that is often and incorrectly worn as protection in a chemical environment is the type usually called “safety glasses”.
These are similar in appearance to ordinary spectacles and could be used in a chemical environment only if it were certain that the only hazard would be from flying fragments, not splashing liquids. In the ANSI standard, these are classified as types A, B, C, and D; the latter three have side-shields that offer partial protection against flying fragments approaching from the side. Type A only protects against a direct frontal flying fragment. None of the four, not A, B, C, or D, offer sufficient protection against splashes of liquids.
All four, however, if they conform to the ANSI standard, is marked Z87. There are two types of “safety goggles”, types G and H, with no ventilation and with indirect ventilation, respectively. Only these two types are suitable for eye protection where chemicals are used and handled. Both types G and H are equipped with flexible edging so that they fit against the skin and thus protect from both flying fragments and flying splashes of liquid from all directions. Make sure that the type G or H safety goggles you and your students use are marked “Z87”.
The first chemical hazard to be discussed is flammability. Although one chemical may indeed be more flammable, say, than another,3 the precautions and emergency treatment depend principally upon flammability itself, not the degree of flammability. A flammable chemical (obviously) will burn. Other terms that convey the same hazard potential information include “extremely flammable” and “combustible”. Keep in mind that the vapours of flammables, if ignited when mixed with air in suitable proportions (ranging from 1% to more than 50% [by volume] in some cases) can explode.
Flammable solids sublime; hence, their vapours are just as hazardous as the vapours from a flammable liquid. For example, glacial acetic acid (solid or liquid, depending on the temperature) is a flammable chemical as defined here. Keep in mind also that the vapours of most flammables are denser than air and can travel 10, 20, or 30 feet, or even further.
The travelling vapours mix with air as they move. Consequently, a source of ignition can be several tens of feet away from the flammable liquid and still cause a fire or explosion by igniting the vapor trail that has traveled from the flammable liquid to the ignition source Precautionary measures include the enforced absence of ignition sources, such as lighted burners, hot plates, other hot surfaces (a lighted incandescent light bulb), and sources of sparks (electrical sparks, static charge sparks, and friction sparks). Keep containers closed when not actually in use.
Ensure that the air movement in the laboratory is sufficient to keep the concentration of the flammable vapour in the air well below 1%. Minimize the quantities available— usually, 100 mL is more than ample for lab use. If more is necessary, provide it in separate containers, 100 mL maximum in each container. Store flammables in an approved flammable liquid storage cabinet, preferably in safety cans. Use fabric, not plastic, tape to tape glass vessels (test tubes, flasks, beakers) beforehand if they are to contain flammable gases or vapours.
Otherwise, when handled by students or used by teachers in demonstrations of an exploding gas or vapour, there can be flying glass shards from the ignition of the air-gas mixture. Even with the necessary taping, conduct such demonstrations only behind a sturdy shield that will confine flying fragments. Ask your local fire department to review your procurement, receiving, storing, handling, dispensing, use, and disposal of flammables and to make recommendations for improving safety.
Make certain in advance that the safety shower is working and that students know how to use it. Ensure beforehand that fully charged fire extinguishers are available that you (not the students) know how to operate, that there has been a recent, successful fire drill, that the fire alarm system is operating, that all persons know what the fire alarm bell sounds like, and what to do when it sounds.
Students should be taught the “stop, drop, and roll” technique to be used if their clothes catch on fire elsewhere and in the laboratory taught to walk calmly to and use the safety shower to extinguish clothes that are on fire. A drill to practice these exercises is recommended
A corrosive chemical either destroys living tissue or causes a permanent change in such tissue through chemical action. (A chemical that corrodes iron, for example, wet salt [sodium chloride], is not corrosive under this definition—which pertains to chemical safety. Sulfuric acid will corrode iron but is also a corrosive in this safety context.) Corrosives can destroy both skin and tissues underneath the skin; corrosives destroy eyes, the respiratory system, and any other living tissue.
Corrosive effects include impaired sight or permanent blindness, severe disfigurement, permanent severe breathing difficulties, even death. Usual precautionary measures include preventing contact with skin, eyes, and the respiratory tract. Wear both safety goggles and a face shield. The face shield should be a full-face shield, large enough, and curved, to protect the whole face, neck, and ears; it, too, should bear the Z87 code mark.
Wear gloves made of a material known to be impervious to the corrosive being handled. Be sure the gloves are free of corrosive contaminant on the inside before wearing. If it is likely that bare arms will be splashed, wear sleeve gauntlets made of the same material as the gloves. Use a lab apron, made of a material known to be impervious, large enough, and sufficiently full-tailored to protect the clothing.
- The apron should be tied so as to protect the lower neck/upper chest and be long enough to protect the calf of the leg. Never wear shoes with open toes, or with woven leather strips, or other gaps over the toes, or with cloth-covered toes in the laboratory.
- Always store corrosives below eye level.
- After handling corrosive chemicals, always wash thoroughly using plenty of water. Promptly flush splashes of corrosives off the skin with copious flowing water for at least 15 minutes.If splashed on clothing, the clothing must be removed while under a safety shower. Do not remove the clothing and then get under the shower.
While under the shower, remove all clothing, including shoes, socks, wristwatch and strap, and other jewelry5 if they are splashed with corrosives (this is no time for modesty). Stay under the shower for at least 15 minutes while someone else calls a doctor. (It helps if the water is tepid, not cold.) Make certain in advance that the safety shower is working and that students know how to use it. A splash of a corrosive chemical in the eye is a very serious matter. Get the victim to an eyewash fountain within 30 seconds maximum, preferably even sooner.
The eyewash fountain must be capable of delivering a gentle but copious flow of fresh water (preferably tepid) for at least 15 minutes to both eyes. (Most portable eyewash devices cannot meet this requirement.) Ensure in advance that safety showers and eyewash fountains are working and that students know how to use them. While the victim is flushing the eyes for at least 15 minutes, someone else should call the doctor for further instructions. (Is the doctor’s phone number already posted by the telephone?)
The victim should hold both eyelids open with thumb and forefinger and roll the eyeballs up, down, left and right, continuously, so as to work the flushing water around to the back of the eyeball and wash any chemical away from the optic nerve. If the chemical destroys a portion of the optic nerve, permanent blindness ensues.
If instead, the chemical destroys a portion of the front of the eye, the prognosis is less pessimistic. In all cases of contact with corrosives, take the victim to a physician for further evaluation and treatment. Irritants are chemicals similar to corrosives except that they do not destroy tissue by chemical action. Irritants cause inflammation, itching, and so on. The effects are usually reversible but may or may not be severe or long lasting; victims should be referred to a physician.
Finally, some chemicals are sensitizers. The first exposure does not usually cause any notable symptoms. The second, or perhaps the third or fourth or more, exposure does cause symptoms because the victim has been sensitized by prior exposure(s). Poison ivy is an example of this kind of effect; some victims can be exposed several dozens of times before that next, and then often quite serious, exposure incident. From the above discussion, it would seem that the use of corrosive chemicals in grades 7–12 should be severely limited or perhaps not used at all.
Corrosive chemicals are potentially seriously harmful. There is no need for their use in pre-high school laboratory work. At that level, purchase and use diluted solutions of the strong acids and bases.6 Other corrosives such as elemental bromine are not needed at all. On the other hand, high school students can use corrosives if the precautions described above are followed. After all, as adults in training, older students can profit from supervised instruction in matters that would be inappropriate for less mature students.
Broadly speaking, there are two different toxic effects, chronic and acute. A chronic toxic effect is noted only after repeated exposures or after a single, long exposure. Commonly known chronic toxic effects include cancer and reproductive malfunctions. Acute toxic effects occur promptly upon exposure, or within a short time—a few hours at most.
Methyl and ethyl alcohol are examples. Both exhibit the same acute toxic effect: inebriation. Ethyl alcohol exhibits a chronic effect: cirrhosis of the liver. Methyl alcohol exhibits two additional acute toxic effects: blindness and death.
To understand this, consider the “dose-response” phenomenon, a characteristic of all toxins, both acute and chronic: the greater the dose, the more severe the response to the toxin. Thus, a very small amount of methyl alcohol inebriates, a bit more causes blindness, yet a bit more is fatal. All toxic substances share this characteristic; exposure to a larger amount of the toxin is worse than exposure to a smaller amount; an exposure of longer duration has a greater toxic effect than the exposure of a shorter duration.
One precautionary measure for toxins is now obvious: Minimize the exposure. Use the smallest amount of a toxin that is suitable for the purposes of an experiment. Minimize the time an experimenter will work with a toxin. Work with toxins only in a fume hood that is known to be operating properly.
Toxic chemicals can enter the body in five different ways, called “routes of exposure”. The first route of exposure to be discussed is inhalation. 1. Inhalation. It is commonly thought that if you cannot smell a toxin, then you are not being exposed unduly. This is true for some odoriferous toxins and false for others; there is no way to tell which is which. It is especially incorrect to think that the more offensive the odour, the more toxic the substance.
The safe procedure is to keep the concentration of the toxic vapour well below the “threshold limit value” (TLV). Not all toxins have been assigned a TLV value. TLVs for toxic chemicals, if a value has been assigned, are given in the MSDS for those chemicals. Note that TLV values pertain to fully grown adults. Younger persons may be more susceptible to toxic exposure than fully grown adults. Therefore, in laboratory work for students in grades 7–12, it is particularly important to ensure that vapour and dust concentrations of toxins are maintained well below established TLV values.
Fortunately, there are many chemicals used for laboratory work in grades 7–12 for which this need not be a concern because their TLV values are sufficiently high so that their expected air concentrations are well below their TLV values. Under certain circumstances, your employer is required to measure the concentrations of toxins in the air you breathe and to provide you, the teacher–employee, both with the results of the measurement and with consultation by a physician or other health practitioner, all at no expense to you.7 No similar requirements have been promulgated for the protection of students or other nonemployees.
The preceding discussion has emphasized inhalation. The other four routes are 2. Injection, for example, by a cut from contaminated, broken glassware or sharp knife. 3. Absorption through intact skin, for example, phenol splashed on the skin—which can be fatal if not promptly flushed off. 4. Ingestion, for example, swallowing a toxic solution. 5. Via other body orifices, such as the ear canal and the eyeball socket. Our eyes are a bit loose in their sockets.
Vapors, mists and fine dust can enter the body via this route. In addition to minimizing the exposure by using the least amount necessary for the shortest possible period of time, precautionary measures for toxins include barriers, cleanliness, and avoidance. Thus, one avoidance precaution is, simply, good ventilation throughout the laboratory as well as the use of fume hoods. Wearing impervious gloves is an example of a barrier precaution.
Cleanliness includes good housekeeping practices, such as minimizing dust from solid toxins, the mist from liquid toxins, prompt spill cleanup, and probably most important of all, thorough washing of hands and arms and scrubbing under fingernails as a habitual practice before leaving the laboratory. Further precautions involve your awareness of the most likely symptoms of toxic overexposure: headache, nausea, and dizziness. Whenever you experience any of these three while you or someone else nearby is working with a toxic chemical, get to fresh air immediately and do not return until the symptom has disappeared.
If on your return the symptom recurs, leave immediately and call a physician; it is likely that you have been overexposed. However, the absence of these or other symptoms does not necessarily indicate no exposure. In advance, read the MSDSs for the chemicals you and your students will be handling. Consult with a local physician in advance, advising him or her of the toxic chemicals used in the lab and ensure that the physician will be prepared in advance to treat victims of toxic exposures. For each toxic chemical, after reading the MSDS: 1.
Evaluate the toxic risk posed to your students in their use, with precautions, of the chemical; 2. Evaluate the educational benefit to be gained if the chemical is used, with precautions, by your students; and 3. Based on the balance between risk and benefit, decide whether or not to use the chemical. And, if you decide to use a particular chemical, be sure that you know • whether or not, in case of ingestion, vomiting should or should not be induced, • the symptoms of exposure to that chemical, and • if applicable, the recommended procedure in case of unconsciousness.
Next, reactive hazards. Container labels do not always describe the fact that a chemical is self-reactive, for example, that it will spontaneously explode, or that if mechanically disturbed it could explode. Nor do labels always state that a chemical, if mixed with certain other chemicals, will react rapidly and release a large amount of energy. For reactivity information, refer to the MSDS for a chemical; if applicable, that information should be described in the MSDS.
Precautionary measures for self-reactive chemicals include, of course, not providing students with any such chemicals. These include picric acid, wet or dry (when dry—as it may become in students’use—picric acid can detonate when mechanically disturbed). Peroxide formers are similarly hazardous. They include metallic potassium, diethyl ether, and other ethers such as dioxane and tetrahydrofuran; their peroxides are explosively unstable when mechanically disturbed. The other reactive hazard is reactive incompatibility.
Even dilute acid is reactively incompatible with the dilute base. Other combinations include oxidizing agents and reducing agents—chlorates and powdered metal, to cite one example. There are other kinds of incompatible pairs. For this, the MSDS is the usual information source.
Precautionary measures include providing reactively incompatible pairs to students only when that provision is deliberately determined by the teacher— and even then providing very small quantities, and only under direct supervision. Precautionary measures also, and emphatically, include proper storage practices. Incompatible pairs are kept separate from each other in the storage area. Above all else, never store chemicals in alphabetical order by name.
Alphabetical storage leads inevitably to adjacent positions for several pairs of incompatibles.8 Chemicals that are incompatible with common fire-fighting media—water, carbon dioxide—should be stored under conditions that minimize the possibility of reactions should it be necessary to fight a fire in the storage area. Refer to the MSDS for information on this incompatibility.
Some of the commercial suppliers of laboratory chemicals for schools have incorporated the use of colour-coded labels with different colours, or alternating stripes of colour, or both, on the label to indicate the manner of storage. Each different colour or stripe code signifies a separate storage space; only chemicals with the same colour or the same stripe coding are compatible with each other and therefore may be stored with other similarly coded chemicals.
When this storage protocol is followed, incompatible chemicals are well separated from each other. Unfortunately, different laboratory chemical suppliers use different colour codes. When storing chemicals from different suppliers, be aware that a chemical coded with a green stripe, say, from supplier X may or may not be compatible with a green stripe-coded chemical from supplier Q. Consult both suppliers’MSDSs for clarification
We come now to our last hazard category, physical hazards. Some physical hazards are associated with chemicals, some with objects, and some with people. A physical hazard that once was quite common among teachers of chemistry was their tendency to accept donations of chemicals from well-meaning donors. An example of a physical hazard that is associated with some chemicals is slipperiness. Concentrated sulfuric acid is very slippery.
It is reported to be impossible to remain standing in the middle of a spill of this acid. Radiation from radioactive species is a physical hazard. Dry ice can cause freeze burns and is another example of a chemical with a physical hazard. Various nonchemical physical hazards include loose clothing (sleeves, blouses, neckties), loose long hair, bulky jewellery, horseplay, hot surfaces, and unattended but still-lit Bunsen burners. Readers can supply their own additional examples. For all of these, the precautionary measures are obvious.