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Fumigants are toxic to humans as well as to insects. The claim may occasionally be made by unqualified or irresponsible persons that a certain fumigant is poisonous to insects but harmless to humans. From their very nature as volatile, penetrating and toxic chemicals, all materials used as fumigants can, if not used with proper precautions, poison human beings. Any exposure before, during or after a fumigation treatment can be harmful; hence anyone using fumigants should have some knowledge of their toxic properties and should take every precaution to avoid exposure to them. Nevertheless, if proper care is taken, the work is no more hazardous than any other industrial or domestic technique that uses potentially harmful chemicals.
In this chapter some of the hazardous features of toxic gases are discussed together with general precautions for handling them and protective measures and equipment used to avoid hazards and detect their presence. Special precautions or considerations applicable to individual fumigants or fumigation procedures are given in the pertinent sections of this manual (particularly Chapter 6).
When handling and applying fumigants it is essential to know for each fumigant the level of concentration above which it is not safe to subject workers and also the maximum periods of exposure, including repeated exposures during normal working hours. Such concentrations are widely known as threshold limits and are usually, and most usefully, expressed in terms of parts per million by volume in air (see Chapter 2 for discussion of methods of expressing concentrations). These threshold limits are published from time to time by responsible authorities or professional organizations in different countries. A comprehensive list for repeated daily exposure is published periodically in the United States by the American Conference of Government Hygienists and may also be found in journals, for example the Archives of Environmental Health, published bimonthly by the American Medical Association.
The purpose and useful interpretation of these limits may best be explained by direct quotation from the most recently published list (ACGIH, 1981):
"Threshold limit values refer to air-borne concentrations of substances and represent conditions under which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect. Because of wide variation in individual susceptibility, however, a small percentage of workers may experience discomfort with some substances at concentrations at or below the threshold limit; a small percentage may be affected more seriously by aggravation of a pre-existing condition or by development of an occupational illness."
Threshold limits are based on the best available information from industrial experience, from experimental human and animal studies and, when possible, from a combination of the three. The limits should be used as guides in the control of health hazards and should not be regarded as fine lines between safe and dangerous concentrations. The pertinent threshold limits set by this American Conference are given in Table 7 along with a summary of odour thresholds for a number of commonly used fumigants.
Two categories of threshold limit values are given in the Table:
Threshold Limit Value-Time Weighted Average (TLV-TWA), i.e. the timeweighted average concentration for a normal eight-hour work-day or 40-hour work-week, to which nearly all workers may be repeatedly exposed, day after day, without adverse effects.
Threshold Limit Value-Short Term Exposure Limit (TLV-STEL) - the maximum concentration to which workers can be exposed for a period up to 15 minutes continuously without suffering from irritation, chronic or irreversible tissue change or narcosis of a sufficient degree to increase accident proneness, impair self-rescue, or materially reduce work efficiency, provided that no more than four excursions per day are permitted, with at least 60 minutes between exposure periods, and provided that the daily TLV-TWA also is not exceeded. The STEL should be considered a maximum allowable concentration, or ceiling, not to be exceeded at any time during the 15minute excursion period. The TWA-STEL should not be used as engineering design criterion or considered as an emergency exposure level.
It is important to realize that if any TLV is exceeded, a potential hazard from that substance is presumed to exist.
Under some conditions, chloropicrin may be used effectively as a prewarning gas in structures, such as ships, where stowaways may be concealed, or in large buildings that are difficult to inspect completely. The prewarming gas is applied separately between 15 to 30 minutes in advance of the main fumigant. A dosage recommended for chloropicrin as a prewarning gas is 6 g/100m³ (1 oz/15 000 ft³). It may be applied by soaking the required amount in a cotton or glass wool pad in a glass or metal pan placed in front of a circulating fan to hasten evaporation.
The corrosive and phytotoxic properties of chloropicrin may be important even with the small amounts used for warning purposes.
Harmful effects from exposure to a toxic gas may fall into two general categories - acute and chronic.
Acute effects can result from a single exposure to high levels of a fumigant, with symptoms generally appearing within a few minutes or hours. The symptoms will vary with different fumigants and different individuals may be affected differently.
Table 7. - ESTIMATES OF ODOUR THRESHOLD AND MAXIMUM EXPOSURES BELIEVED SAFE FOR HUMAN SUBJECTS (values given in ppm).
|Approx odour threshold||
|Carbon disulphide||< 1||10||-|
|Ethylene dibromide(2)||25||( )||-|
1 Data on threshold of odour from different sources often differ considerably because evaluation of smell is subjective and somewhat variable. The data on odour threshold should be regarded only as a guide.
2 Listed as human carcinogens by the American Conference of Governmental Industria Hygienists (ACGIH, 1981). Parentheses indicate proposed values.
3 Listed as "substances suspect of carcinogenic potential for man" (ACGIH, 1981).
4 The odour associated with phosphine appears to be caused by impurities, which can be separated from phosphine under some conditions(Bond and Dumas, 1967; Dumas and Bond, 1974).
Chronic or long-term effects may result from an overdose on a single exposure to a toxic gas or from exposure to low levels over a period of time. The effects may not appear until long after exposure and in some cases they may not be easily associated with the poison. It has been demonstrated that certain of the fumigants can produce cancer in test animals under experimental conditions. More detailed information on these and other hazardous properties of individual fumigants are given with each fumigant in Chapter 6.
Some fumigants can be absorbed through the skin (including mucous membranes and the eyes) either from the gas or more particularly by direct contact with the substance. When materials are absorbed in this way the threshold limit values given in Table 7, which refer to airborne concentrations of substances, may be invalidated (ACGIH, 1981).
General precautions for the handling and use of all fumigants may be considered under four main headings: preliminary, those taken during application, those taken during the exposure period and post-treatment.
Under this heading may be listed advance precautions of a more general or permanent nature and also preliminary measures applying specifically to each fumigation job.
In any fumigation, large or small, no person should work alone. Because poisonous gases are being used, serious consequences may ensue if a fumigator becomes sick or faint and is unable to finish or control the operation. No matter how small the dosage or the scale of the work, at least one other person should be present in case of emergency.
PRECAUTIONS DURING APPLICATION
In addition to proper respiratory protection, scrupulous care should be taken to ensure that fumigant formulations or liquids do not come into contact with the skin. If clothing or footwear becomes contaminated, it should be removed immediately and affected areas of the skin washed thoroughly with soap and water.
PRECAUTIONS DURING EXPOSURE PERIOD
Every precaution should be taken to avoid exposure to escaping fumigant and to prevent unauthorized entry into treated space. Warning signs that indicate the type of fumigant in use and the date of the treatment should be put in appropriate places. They should be removed after the treatment is completed.
After adequate aeration of the treated area, gas detection equipment should be used to ascertain that all fumigant has been removed. As the process of desorption can vary greatly with different fumigants, different commodities and different environmental conditions, precautions should be taken to ensure that harmful levels of gas do not subsequently accumulate where personnel could be exposed.
All members of fumigation crews should be thoroughly trained in basic first aid, with additional emphasis placed on artificial respiration techniques for gas poisoning. Such training is valuable not only for its immediate practical purpose, but also because it serves to emphasize the need for care in all aspects of fumigation.
FIRST AID KIT
All persons engaged in pest control work should carry with them, or have access to, an adequately provisioned first aid kit. Included in this kit should be pertinent information on the nature of poisoning by fumigants or other pesticides used, together with suggestions for remedies. This information would be especially useful to a physician called in to treat an emergency case.
It is essential that those regularly engaged in fumigation be under proper medical supervision. The physician should be fully informed of the chemicals used and the manner of their application. Regular medical examinations will check the general health of the operators and reveal the appearance of conditions that may require affected personnel to be removed from this type of work either temporarily or permanently.
INFORMATION FOR PHYSICIANS
It cannot be overemphasized that physicians concerned should be supplied in advance with details of the fumigants used and their toxic effects. The preliminary symptoms of poisoning by toxic gases may be the same as for other common complaints. In addition, occurrences of fumigant poisoning are rare and the average physician is glad to be informed in advance of symptoms and remedies for a condition not ordinarily encountered in common practice.
In cities or areas where public hospital facilities are available, it is usually possible to advise the hospital authorities of the possibility of accidental poisoning by fumigants. The staff of the emergency departments may then make arrangements for suitable antidotes to be on hand.
POISON CONTROL CENTRES
Poison control centres are being organized in some countries, and in future they will no doubt become increasingly available in all parts of the world. These centres have been created primarily to deal with the alarming increase in the number of accidents, especially among young children, caused by swallowing the many poisonous substances now available to the public. However, their services may be utilized in emergencies arising out of industrial procedures, such as fumigation work. From these centres physicians may obtain current information on the symptoms and treatment of poisoning. These centres may stock antidotes and even give emergency treatments. In providing for proper medical supervision of their work, fumigators should ascertain, either directly or through their own physician, if there is such a contra situated in their own area and, if so, the nature and extent of the service provided.
If antidotes are recommended as first aid measures for any of the poisons used, they should be purchased and carried in the first aid kit. Even if the antidote may be administered only by a qualified physician, it should be placed in the kit together with any needed accessories, such as sterile hypodermic needles and up-to-date information on the amounts of antidote required, and at what intervals of time. Specific treatments are discussed in detail in Chapter 6. General first aid measures in case of accidents caused by the inhalation or spilling of fumigants are given in Appendix 3.
The respirator is the most important piece of equipment used for the protection of persons working with fumigants. When fumigation is carried out regularly, it is advisable for each of the operators to be supplied with his own respirator so that he himself is responsible for its care and upkeep, for his own personal protection.
The only respirators that should be purchased for fumigation work are those approved for the purpose by a recognized government authority, such as the mines or public health departments, in the country of purchase or manufacture. Such approval usually extends only to a complete assembly. If a certain make of respirator is purchased, the canisters used with it should be obtained from the same manufacturer.
The term respirator is used in many English-speaking countries to describe a device whereby the entire face is covered, or the nose and mouth alone are enclosed, so that the wearer may breathe only filtered air from the surrounding atmosphere; these devices are also called gas masks. Breathing takes place through a filter, which is designed to remove certain contaminants, or through a hose that draws fresh air from outside the space being fumigated. There are also two types of closed circuit respirator available:
- a self-contained unit, using compressed air, carried on the person in one or more small cylinders (bottles);
- a type of self-generating apparatus whereby the oxygen is evolved from a special canister by the action of moisture from the breath.
For most fumigation work the most convenient type of respirator is one that employs a Filter-type canister. This is usually referred to as the industrialtype respirator or gas mask; one type is shown in Figure 8. The canister on this type of mask gives adequate protection for a certain length of time from gases that do not exceed a concentration of 2 percent by volume in air (0.5 percent for phosphine); it contains a chemical or physical adsorbent designed to remove contaminant gases from the air being breathed. Canisters are designed to prevent the passage of a particular gas or group of gases. It is most important to check before each fumigation that the canister on the respirator is the right one for use with the specific gas or mixture of gases that will be used for that particular job.
When an operator is applying fumigants and is likely to be working close to the point of emergence of the fumigant from the container, it is good practice to wear the canister on the back, as in Figure 8. This is particularly important when using liquid-type fumigants, as illustrated.
In certain types of fumigation work, such as the spraying of liquid fumigants over large masses of bulk grain, the self-contained types of breathing apparatus, air line masks or the safety blouses (shown in Figure 9), which draw fresh air from outside the building, may be used.
Cartridge-type respirators are small devices with one or two small chemical cartridges attached to the nosepiece. These are usually designed to give protection only against gases up to 0.1 percent by volume. They should not be used in any phase of fumigation work. Also, respirators designed as dust filters, or for use with insecticidal or fungicidal aerosols, afford no protection whatsoever against fumigants. The specific types of canisters recommended for use with particular fumigants or groups of fumigants are listed in Table 8.
When wearing a respirator, a person with punctured eardrums may draw fumigant vapours in through his ears as a result of creating a slight negative pressure during inhalation. Any fumigant drawn this way will be exhaled into the inside of the respirator facepiece, and a poisonous concentration may build up inside the respirator. It is usually possible for persons with this defect to obtain complete protection by using cotton earplugs covered with oil.
Men with beards usually cannot fit respirators tightly enough to the face for adequate protection.
Use and Care
Detailed instructions for adjusting, putting on and checking respirators are supplied with each unit purchased. These printed instructions are usually placed inside the lid of the carrying case or in some other convenient place. They should be carefully studied at the time of purchase and read over again before the respirator is used. Supervisors should give new operators detailed instructions on the proper use of the respirators. If a person has not worn a mask before, a regular daily drill should be undertaken to rehearse the proper procedure and movements. This drill should be continued until the new fumigator can demonstrate full familiarity with the correct handling and use of the respirator.
Table 8. - TYPES OF CANISTERS USED WITH RESPIRATORS RECOMMENDED FOR RESPIRATORY PROTECTION AGAINST FUMIGANTS.
|Compound||Designation of Canister Type||Remarks|
|Carbon dioxide||AG||oxygen in atmosphere should not be less than 16 percent|
|Carbon disulphide||OV||skin penetrant|
|Carbon tetrachloride||OV||skin penetrant|
|Ethylene dibromide||OV||avoid skin contact|
|Methyl bromide||OV||skin penetrant|
|l,l,l - Trichloroethane (Methyl chloroform)||OV|
|Hydrogen cyanide||AG||absorbed through skin|
|Hydrogen phosphide||PHOV||not to exceed 200 ppm; above this level supplied air to be used|
AG - Acid gases
OV - Organic vapours
OVAG - Organic vapours and acid gases
PHOV - Phosphine, organic vapours
The canister is that part of the respirator that actually removes the poison from the air breathed in. Therefore its use and limitations must be understood.
Contents and Capacity
Industrial types of canisters which are recommended for fumigation work may contain three kinds of materials:
1. Activated charcoal to adsorb organic vapours, such as methyl bromide, ethylene dichloride and carbon tetrachloride.
2. Chemicals to react with certain gases; for instance, soda lime, which neutralizes acid gases such as HEN and sulphur dioxide.
3. Cotton or other filters to remove dust.
As mentioned above, the manufacturers state on each canister that it is not to be worn in concentrations above 2 percent by volume of the gas in the air. With phosphine, however, the maximum concentration is 200 ppm.
At or near this maximum concentration, which is above fumigant concentrations normally used, the canister will not afford protection for more than 10 minutes. When fumigants such as HCN and chloropicrin, which have a distinct smell, are being used, the operator is warned of the exhaustion of the canister by a slight odour characteristic of the fumigant. Methyl bromide has no odour at comparatively low concentrations and the special precautions needed are included in the discussion of this fumigant.
The possibility of the revivication of canisters containing the activated charcoal used for organic vapours is discussed at the end of this section.
Use and Care
When a canister is new its top and bottom are sealed. Manufacturers stamp an expiry date on the label in order to indicate when the canister must be discarded even if the seals have not been broken.
The supply of canisters should be stored in a cool, dry, well-ventilated place away from contamination by any gases. Before use the following precautions should be observed:
1. When the canister is attached to the respirator after the top seal is removed, the date should be recorded. This is best done by writing the date on a small linen label, which can be tied to the respirator harness near the canister. This label can be used to record the exposure of the canister to the fumigant.
2. If the canister is not exposed to poison gas after the cork is removed, it may be retained in the respirator for one year - but not longer - if stored as above.
3. Before the respirator can be used, it is necessary that the cap or seal over the air inlet valve of the canister be removed. Again, at this time the date should be marked on the label. Once this seal is removed, even if there is no exposure to fumigant, the canister should be replaced after a lapse of six months.
4. After every fumigation operation in which there has been exposure to the gas, the canister should immediately be discarded. When high fumigant concentrations are encountered in the work, application and aeration should be considered as separate operations, and after each, a fresh canister should be put on the respirator.
On exposure to lower concentrations, which might be encountered during the aeration or inspection of fumigated structures, the canister should be replaced after two hours, as shown by the label. A wide margin of safety should be allowed in estimating exposure times. Canisters cost little in terms of the health of the individual. If there is any doubt about the exposure life of the canister it should be discarded (see special considerations for methyl bromide in Chapter 6).
5. In addition, canisters should be discarded when any of the following conditions prevail:
- external damage - a severe blow may cause displacement of the contents, permitting contaminated air to pass through to the wearer;
- detection of increased resistance to inhalation - excessive moisture uptake by the canister can impede air flow;
- if lens fogging occurs and fails to clear on inhalation;
- the expiry date is past.
6. Immersion of the canister in water renders it useless. Water may enter the canister through the facepiece, so care should be taken that no water enters the hose connexion while the respirator is being cleaned or disinfected.
7. When canisters are discarded, all labels on them should be clearly marked with indelible pencil or black wax pencil "Exhausted" or "Used up". They should be destroyed and immediately sent to the refuse dump under conditions which will prevent them from being picked up and used again.
FINAL CHECKS ON RESPIRATOR
Before any person enters a space where the atmosphere contains a fumigant or undertakes any procedure calling for the use of the respirator, several important points should be checked; these are enumerated below. Also, 8 thorough physical check should be carried out on the proper working of the respirator.
Important Points to Check
1. Is the right canister being used?
2. Is the highest expected fumigant concentration within the absorbing capacity of the canister? (As already stated, industrial canisters are designed for use in gas concentrations not exceeding 2 percent by volume in air. In the tables accompanying the more important fumigants, this value is given in terms of g/m³ or oz/l 000 ft³ ).
3. Is the canister in fresh enough condition to give the protection desired? The answer to this should be provided by the record kept on the tag tied to the canister.
4. Even if proper respiratory protection is being given, is there a possibility of gas absorption through the skin? (This consideration was discussed above. Among fumigants, it applies principally to HCN but is also considered later when the different fumigants are discussed).
5. Is there enough oxygen present in the atmosphere to be entered to support normal respiration?
6. Are there other noxious gases in addition to the fumigants? (The ordinary industrial-type canisters used for fumigants will give no protection against carbon monoxide and illuminating gases).
Check for Gas Tiqhtness
After the respirator is put on for actual use with a fumigant, the final check on tightness and proper fit is most important. This is performed as follows:
Place the hand lightly over the air intake at the bottom of the canister and take a deep breath. If the respirator is properly adjusted, a strong draught of air will be felt as it enters the canister.
If the canister is connected to the facepiece by a hose, pinch the hose off tightly with the hand. If the facepiece is fitting tightly and properly and there are no air leaks, the wearer will not be able to breathe.
If there is no hose, place the hand firmly over the canister intake so that no air can enter. If the respirator is fitted properly the wearer will not be able to breathe.
REGENERATION OF EXHAUSTED CANISTERS
In some countries fresh respirator canisters may be difficult to obtain on account of the problems of supply or foreign exchange. On general principles, as stated above, new canisters should be used for each fumigation job but in an emergency canisters containing activated charcoal only to remove the fumigant from the inspired air may be revived by a regeneration process (Muthu et al, 1964; Maggs and Smith, 1975). If the regeneration process is carefully followed with appropriate testing, it is considered that revived canisters are safe and may be reused for methyl bromide and possibly other fumigants. With the passage of time, however, the activation of charcoal in the canisters may decline and therefore it is again necessary to stress the importance of adequate testing. For information on regeneration procedures and precautions, the reader should refer to Muthu et al (1964) and Maggs and Smith (1975).
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