Human Factors Focus on Warnings, Part l: Labels, Signs, and Tags

TASA ID: 568

Failure to warn has become a common cause of action in products liability and tort litigation. Over the past 15 years, much scientific research has been conducted on the subjects of warning design and effectiveness.  As discussed here, a warning is a label, sign, or tag used to communicate hazard information. (Note: Part II, a sequel, will discuss visual and auditory warning devices and alarms.)  The purpose of a warning is to modify human behavior and ensure safety compliance, i.e. to give the worker or user an opportunity to avoid harm.  Thus, a warning is distinguished from instructions which focus on efficiency in use of a product.  There is overlap here when warnings are included in user instruction manuals for assembly, operation, and maintenance.  Desirably, on-product warnings should be graphically duplicated in the text of such manuals.

During the product design process, a warning should be treated as a product itself, and receive the same serious attention in its design as the item to be manufactured.  Thus, a poorly-designed warning can be regarded as a design deficiency. Relative to product safety design, the use of warnings should be considered only as a last resort, since their very presence on a product may indicate the manufacturer has failed to address a product design hazard. Good safety design practice recognizes a three-step hierarchy:  (a) design out the hazard, when feasible, in the first place; (b) if not feasible, guard against exposure or otherwise protect the user; and (c) only then use warning labels, signs, or tags.

The most critical prerequisite for an effective warning is that it gains a person's attention. Such elements as location of a warning on a product, illumination, type size and font, and color are important in this regard. Good warning format includes the following: (1) an alert symbol - exclamation point with a triangle - followed by an appropriate signal word and colored background, i.e. red/danger, orange/warning, and yellow/caution; (2) Condensed verbiage describing (a) the nature and severity of the hazard, (b) the consequence of exposure for people, (c) action to be taken to avoid the hazard, and sometimes (d) procedure to treat exposures.  Since people are unlikely to read lengthy text, brevity and comprehension become critical.  Pictorials often are a desirable adjunct to verbiage, especially for communication with non-English speaking people.  Complete sentences are unnecessary here. Wording must be explicit, e.g., "corrosive" fails to specify the hazard to people; rather than vague "adequate ventilation," specify air movement in CFM or "respirator use mandatory."

The definitive standard on warning labels, signs, and tags is the ANSI Z 535 series - five parts in all.  Other warning standards exist and apply to specific hazard situations; for example, fire (NFPA), hazardous waste (40CFR), chemical shipments (49CFR), industrial chemicals (ANSI Z 129.1), and consumer products (16 CFR).  With so many standards, it is not surprising that an industrial product may be covered with numerous warnings which compete for attention and confuse the viewer.  Then too, one may encounter countervailing, representations wherein an "environmentally friendly" label misleads one's interpretation of a hazard warning.

Focus on Warnings, Part II - Visual and Auditory Warning Devices and Alarms

Warning devices, as discussed here, exist to alert and inform people regarding the presence of hazards, operational states, and system failures or malfunctions.  An effective device or alarm should capture attention, provide some indication of a problem, and desirably suggest some action.  From a human factors engineering perspective, relevant design factors include such issues as information overload, the task environment, human reaction time, and choice between visual and auditory signal parameters.

For humans to detect visual or auditory signals, there must be good contrast (e.g. the brightness of a target against its background, a sound amid noise).  Note that stars are always in the sky to be "seen," but are only perceived under appropriate contrast conditions.  Beyond detection, there is the need to distinguish a warning from other signals - to discriminate among such design parameters as intensity (brightness, amplitude); color (wavelength);  auditory frequency; and signal duration, complexity and location.  The use of visual devices is limited by the field of vision; desirably a warning light is located within 15 degrees of a task's field of view (automobile turn indicators and high beam indicators are commonly poor in this regard).  Auditory devices possess the benefit of omnidirectionality - we don't have to face the signal source - and they are useful when we cannot see (low illumination) or the task involves visual overload. Use of standard (learned) color codes is important (red for emergencies, yellow for impending danger).  Slow flash rates should be avoided; a rate of five cycles per second (on and off times) is better.  For greater visibility and conspicuity at distances, high-intensity strobe lights of brief duration pulses are recommended for such situations as slow-moving and emergency vehicles or those abandoned or loading/unloading in otherwise freely-moving traffic lanes (e.g. buses, tractor-trailers, tow trucks).  Note that blue signal lights can be detected at greater distances in darkness. Some situations require us to be prepared for the unexpected, e.g. the traffic light on the far side of an underpass; here an alerting signal - flashing light - gives advance of a second warning about to appear.  Similarly, in some automobiles, a buzzer alerts the driver to check her instrument panel where she then sees a flashing fuel pump icon indicating 25 miles to "empty."

Both visual and auditory warnings have their limitations.  While we cannot close our ears to sounds, auditory alarms must be at least 10 dB above ambient noise levels, yet not so loud as to damage hearing.  To some degree auditory alarms generally involve quicker reaction times; with visual signals, processing delays can involve head and/or eye movements, changes in visual accommodation and convergence, and more neural processing.  A fine line must also be recognized as to distinctions between attention, distraction, and annoyance. Some warning lights can be blinding to an unintended observer.  Flashing lights on the side of the highway may be those of an emergency vehicle or those "advertising" a tavern. Loud warnings intended for others interfere with communications or otherwise distract and annoy.  Guidelines for continuous, intermittent, and undulating tonal signals exist, as do those for various auditory devices such as buzzers, bells, horns, chimes, and sirens.  Annunciator "displays" (used in nuclear power plant control rooms) combine both warning device models - an auditory alarm alerts the operator to a flashing indicator that displays a written message.

Finally, one must recognize that even the best of warning devices may not receive attention.  Operators may habituate to or "tune out" auditory alarms.  There are situations where we see but not hear - and vice versa.  While listening to a cellular phone message, the driver may fail to see the traffic light.  A crane operator visually focused on his clamshell may not hear a proximity alarm indicating potential contact of his crane's boom with overhead power lines.  While we can drive a car, monitoring the roadway and carrying on a conversation concurrently, there are situations when heavy traffic puts such demands on our limited channel capacity for information processing that we no longer can "hear" the stock market or weather reports over the car radio.  This allocation of attention problem is beyond the scope of this brief article.  Notwithstanding, it is important to note the role which engineering design can play to obviate the need for warnings and human attention.  Contemporary sensor technology may be applied to supplement or complement human sensory abilities, i.e. to detect the hazard and possibly to take action in response thereto.

This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

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