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Types of Dusts and their Effects

Types of Dusts and their Effects:
Dust is a disperse system (aerosol) of solid small particles in air or gas whose size distribution is like a colloid. It originates from mechanical communication of coarser material. Mining, breaking, crushing, grinding, mixing, polishing and handling are the main dust generating processes. 
Small particles of 0.1 to 5 u.m. size (respirable dust) can remain in the alveolar passages of which smaller particles (O.I r-lm) behave as colloids or smoke, deposit in lungs and other parts and cause health effect. Particles or larger size (>5 (im) are driven back by the clearance mechanism. Asbestos fibre of 3 (urn or less in diameter and upto 100 (in length can reach the alveoli, while the smallest fibres can reach upto pluera and pluera space. 
2nd Schedule of the Factories Act prescribes TLV (permissible time-weighted average i.e. TWA concentration per 8 hours) of cotton, asbestos, coal, cement and silica dusts. 
Types of dusts can be classified as under:

A) Quartz and mixtures containing quartz: Coal dust, mineral ore, sand, rock, fluorspar, quartzite etc. They are found in mining, ceramic industries, refractories, pastering, mixing and insulating materials. They cause nodular fibrosis and silicosis. 
B) Asbestos snd mixtures containing asbestos: Raw asbestos, chrysotile, amphibole, asbestos cement talk etc. It is used in some 3000 products of textile, insulation, packing, jointing and building material. It can cause diffuse fibrosis, carcinoma, asbestosis and Telecosis. 
C) Metals and metal compounds: metals like iron, nickel, lead, mangnese, aluminium, beryllium, chromium, cadmium, vanadium and their oxides are extensively used in metallurgy, metal working, welding, electroplating, furnaces, sintering etc. They can cause irritation, diffuse Fibrosis and different types of lung diseases known as siderosis, bronchial carcinoma or asthma, tracheobronchitis, aluminum lung etc. 
D) Plant and animal (Organic) dust: Wood, animal hides, skins, hair, feathers, scales, cotton flax, hemp, sisal, jute, mouldy hay, straw, cereals, bagasse, crushed grain and bran, enzymes etc. They are found at wood working, agriculture, poultry, textile, grain or sugar Mill etc. and cause irritation, immune reaction, carcinoma, allergic rhinitis, bronchial asthma, farmer's lung, bagassosis, byssinosis etc. 
E) Other dusts: These are chemical dust like carbon dust, soot, graphite, phtalic and maleic anhydride and arsenic dust etc. and found in mining, metallurgy, rubber, plastic and chemical industry. They can cause irritation, carcinoma, systemic effect, ulceration, conjunctivitis, graphite pneumoconiosis etc. Inert dusts are also hazardous. 

Dust Control Methods:-

To prevent lung diseases, some control measures areas under:
1.Know the exposure limits of dusts. Dust below 5 microns size is invisible. Depending on toxicity, exposure limits vary from 0.1 to 10 mg/m3 Employ effective measures based on this safe limit and nature of the dust. 
2.Elimination of dusty process e.g. improved casting technique to eliminate dusty fettling process. 


3.Substitution by a less toxic or non-toxic dust, e.g. shot-blasting in place of sand blasting, metal moulds in place of sand moulds and glass fibre or slag wool in place of Asbestos insulation. 
4.Segregation and enclosure of the process of dust generation cannot be prevented. Dusty process should be enclosed in a room and be connected with effective exhaust and dust collector. Complete enclosure is the best segregation, e.g. blasting cabinet, fuming cupboard. 
5. Wet methods prevent particles becoming airborne. Powdered material is suspended or dissolved in a liquid. The correct degree of wetting should be maintained and it should not be allowed to dry out. 
6.Local exhaust ventilation should be applied to collect the dust from the nearest possible distance. The smaller enclosure gap requires smaller exhaust rate. Auction flow should be away from the worker's breathing zone. Dust collection, filtration and disposal are the subsequent steps. Various kinds of air cleaning devices are also available. 

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