creating or supplementing MSDSs. The National Library of Medicine (NLM) and the Chemical Abstracts databases are examples. These and other such databases are accessible through various on-line computer data services; also, most of this information is available as CD-ROM and computer updates. Many of these services can be accessed for up-to-date toxicity information.

3.B.6.1 The National Library of Medicine Databases

The databases supplied by NLM are easy to use and relatively inexpensive. TOXLINE, the best source of information for most people, covers data published from 1981 to the present. For data published in the period from 1965 through 1980, TOXLINE65, a back file of TOXLINE, is also available. The telephone number to call for information and instructions on obtaining an NLM account is 1-800-638-8480.

Other databases supplied by NLM are the Hazardous Substance Data Base (HSDB), the Registry of Toxic Effects of Chemical Substances (RTECS), and the Medical Literature Analysis and Retrieval System (MEDLARS). NLM also supplies other specialized databases called CANCERLIT, DART, GENETOX, IRIS, CCRIS, and CHEMID.

3.B.6.2 Chemical Abstracts Databases

Another source of toxicity data is Chemical Abstracts (CA). In addition to the NLM, several services provide CA, including Knight-Ridder Information (formerly DIALOG), ORBIT, STN, and Ovid Technologies (formerly CD Plus). Searching procedures for CA depend on the various services supplying the database. Searching costs are considerably higher than for NLM databases because CA royalties must be paid. Telephone numbers for the above suppliers are as follows:

Knight-Ridder Information

1-800-334-2564

ORBIT

1-800-456-7248

STN

1-800-848-6533

Ovid Technologies

1-800-289-4277

Specialized databases are available from a vendor called Chemical Information Systems (CIS) for aquatic toxicity, dermal toxicity, EPA TSCA FYI, 8(d) and 8(e) studies, and so on. The CIS telephone number is 1800-CIS-USER.

Searching any database is best done using the Chemical Abstracts Service (CAS) Registry Number for the particular chemical. Free text searching is available on most of the databases except MEDLINE, which has a controlled vocabulary. As mentioned above, a menu-driven format is available to aid the inexperienced user. Equipment needed to do a search includes a computer terminal, a modem for accessing the on-line database by telephone, and a printer. Results of the search can also be captured by using an electronic format (e.g., a floppy disk).

3.B.6.3 Informal Forum

The "Letters to the Editor" column of Chemical & Engineering News, published weekly by the American Chemical Society, has become an informal but widely accepted forum for the reporting of anecdotal information on chemical reactivity hazards and other safety-related information. This publication is accessible via full-text searching services provided by STN.

3.C TOXIC EFFECTS OF LABORATORY CHEMICALS

3.C.1 Basic Principles

The chemicals encountered in the laboratory have a broad spectrum of physical, chemical, and toxicological properties and physiological effects. The risks associated with the use of laboratory chemicals must be well understood prior to their use in an experiment. The risk of toxic effects is related to both the extent of exposure and the inherent toxicity of a chemical. As discussed in detail below, extent of exposure is determined by the dose, the duration and frequency of exposure, and the route of exposure. Exposure to even large doses of chemicals with little inherent toxicity, such as phosphate buffer, presents low risk. In contrast, even small quantities of chemicals with high inherent toxicity or corrosivity may cause significant adverse effects. The duration and frequency of exposure are also critical factors in determining whether a chemical will produce harmful effects. In some cases, a single exposure to a chemical is sufficient to produce poisoning. On the other hand, for many chemicals repeated exposure is required to produce toxic effects. For most substances, the route of exposure (through the skin, the eyes, the gastrointestinal tract, or the respiratory tract) is also an important consideration in risk assessment. In the case of chemicals that are systemic toxicants, the internal dose to the target organ is a critical factor.

When considering possible toxicity hazards while planning an experiment, it is important to recognize that the combination of the toxic effects of two substances may be significantly greater than the toxic effect of either substance alone. Because most chemical reactions are likely to produce mixtures of substances whose combined toxicities have never been evaluated, it is pru-



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