This article throws light upon the top two measures to counteract harmful effects of ultrasound. The effects are: 1. Technical Preventive Measures 2. Medical Preventive Measures.
1. Technical Preventive Measures:
The prevention of the harmful effects of ultrasound and also of noise from industrial ultrasonic equipment relies on several technical preventive measures such as:
ADVERTISEMENTS:
a. Automatic loading and unloading of ultrasonic washing and cleaning tanks;
b. Provision of remote controls which practically eliminate exposure by contact; and
c. Limitation of the time during which workers may be exposed to ultrasound and noise.
Changeover to low-power equipment makes it possible to reduce the intensity of ultrasound and noise by 20-40 dB (in ultrasonic cleaning, brazing and drilling, for example).
This kind of changeover is usually not contrary to technical requirements. Designers of ultrasonic equipment should choose working frequencies as remote as possible from the audible range of frequencies (not lower than 22 kHz) in order to avoid the effects of high-frequency noise.
All ultrasonic equipment operating at noise and ultrasound levels which exceed the permissible ones must be provided with sound-isolating devices (enclosures, shields, etc.) made of steel or duralumin sheet lined with sound absorbing materials (such as rubberoid, synthetic resins or anti-drumming compound).
ADVERTISEMENTS:
Sound- isolating panels on ultrasonic equipment must be free from slots or other openings and they must be isolated from the floor by rubber seals.
The electric connection between the generator and the ultrasound transducer must be shielded to protect the operators from electromagnetic fields. In cases where noise and ultrasound cannot be reduced to the permissible levels by shielding or enclosure, units operating at levels around 135 dB must be located in sound-isolated booths.
In order to prevent contact with liquid or solid media transmitting ultrasonic vibrations, it is important to disconnect ultrasound transducers while performing manual tasks. In other cases, it is necessary to provide special tools with vibration- isolating handles and to protect the hands with cotton lined rubber gloves.
The velocities of vibration in the frequency range 8-2,000 Hz at the surface of ultrasonic tools (e.g., welding guns) and work-holding fixtures must not exceed the values recommended for limiting the harmful biological effects of vibration. If the vibration exceeds these values, the tools and fixtures must be provided with absorbent linings which lower the vibration to the recommended standard.
ADVERTISEMENTS:
Ultrasonic equipment producing noise and ultrasound within permissible limits may be installed in normal premises, and may need no protective devices. If the noise and ultrasound from the equipment exceeds the permissible levels, the equipment must be installed in isolated premises.
However, if the ultrasound equipment must be located in normal premises for technological reasons (e.g., assembly lines), it must be segregated by partitions reaching to the shop ceiling.
In very high shops, on the other hand, such equipment may be enclosed in booths or boxes, so that the noise and ultrasound levels resulting at the workplaces are reduced to permissible limits. Locating ultrasonic equipment in sound isolated premises or booths, and providing it with remote controls, considerably lower the noise and ultrasound levels.
If work has to be performed for short periods on operating equipment producing high levels of noise and ultrasound, personal protection such as ear-muffs must be worn. When chemical products are used in ultrasonic cleaners, measures against their harmful effects must be taken.
ADVERTISEMENTS:
Aromatic hydrocarbons of the benzene series (e.g., benzene and toluene) should not be used as solvents for cleaning and degreasing. On the other hand, if chlorinated hydrocarbons (e.g., dichloromethane and trichloroethylene) are used, appropriate safety rules must be observed.
Workers operating high frequency ultrasonic equipment must protect their hands. In laboratory work involving the use of a liquid, or during underwater massage for physiotherapy, any contact with the liquid must be avoided.
Operators of crack detecting ultrasonic equipment must keep their hands away from the piezoelectric element. They should wear rubber gloves when testing the transducers of crack detectors, or when setting high-frequency generators.
Manufacturers of ultrasonic equipment should ensure that the operating instructions include important data such as the sound pressure levels in the third- octave bands of the working frequencies. The instruction manual supplied with the equipment should also state the operating speeds to which the ultrasound characteristics correspond.
In order to establish the ultrasound characteristics of a unit, measurements must be taken at points of reference 1.5 meters above the floor level, at a distance of 0.5 meter from the unit contour, and not less than 2 meters from reflection surfaces.
These measurements should be performed at not less than four points of the contour. The distance between any two points of measurement should not exceed 1 meter. The highest value measured should be recorded in the test certificates.
2. Medical Preventive Measures:
Workers required to operate ultrasonic equipment should be informed about the harmful effects of ultrasound and the protective measures to be taken. They should be trained to adopt safe working methods.
It is advisable, in this connection, not to employ for work with ultrasonic equipment the persons with following conditions:
(a) Chronic diseases of the central and peripheral nervous systems, neuritis, and polyneuritis;
(b) Systemic and vascular neurosis;
(c) Previous cranial trauma (cerebral concussion);
(d) Metabolic and endocrine disorders;
(e) Diseases of the labyrinth and chronic diseases of the ear;
(f) Permanent hearing loss of any origin; and
(g) Hypotension and hypertension.
Periodic medical examination should be carried out once a year in the presence of a neurologist, a therapeutist, and an otolaryngologist. Conclusions about the hearing functions must be based on audiometric data and whispered speech tests. It is also important to examine the vestibular apparatus.
Persons showing a hearing loss of 20 dB or more between two periodic examinations, or suffering from disorders of the sense of balance must be transferred to work not exposing them to ultrasound and noise.
Complaints about ill-health, asthenia, or symptoms of peripheral dystonia are indicative of the need to transfer the worker to a workplace where he is no longer exposed to ultrasound. In such cases, the worker concerned should be followed up. When this symptoms have disappeared, the worker may return to his former workplace, provided that measures have been taken to limit the ultrasound level.