Restricted substances - Lead
4 June 2008An awareness of restricted substances is critical for all involved in the leather supply chain. Many substances are restricted either through legislation or through the brands and eco-labels. Although the leather industry is becoming more aware of the substances that are restricted, it is of interest to consider the background to their listing and some of the reasons behind their restriction. This article provides an overview of the element lead (Pb).
What is lead? Lead is a post-transition metal which can occur naturally in its native form, although this is rare. The most important lead mineral is Galena. More commonly, lead is found in ore with zinc, silver and copper, so it is often extracted with these metals. When lead is freshly cut it is a bluish-white colour which tarnishes quickly in air to a dull grey. It is well known that lead is a poisonous metal as it can cause nerve damage and blood and brain disorders. Most lead ores contain only 10% lead and can contain significant concentrations of silver, resulting in the smelted metal also containing silver as a contaminant. Metallic silver and gold are recovered. Production and consumption of lead is increasing annually and is now about 8 million tonnes/year of which half is produced from recycled scrap. The toxicity issues Lead poisoning, a medical condition caused by increased levels of lead in the blood, is one of the main issues when considering lead toxicity. Lead may cause irreversible neurological damage as well as renal disease, cardiovascular effects and reproductive toxicity. Unlike some other metals, lead has no known biological role in the body. The toxicity comes from its ability to mimic other biologically important metals, the most notable of which are calcium, iron and zinc. Lead can displace other metals in enzymes and thus deactivate them. Long term exposure to lead or its salts can cause neuropathy and colic-like abdominal pains. The concern about lead's role in cognitive deficits in children has brought about widespread reduction in its use. In the majority of cases elevated blood lead levels in adults are workplace-related. Many older houses may still contain substantial amounts of lead paint and, therefore, old paint should not be stripped by sanding, as this generates inhalable dust. Lead salts used in pottery glazes have on occasion caused poisoning, when acidic drinks such as fruit juices have managed to leach lead ions out of the glaze. Industrial uses Lead is a dense, ductile, soft, very malleable metal and therefore has many uses in industry, a few of which are outlined below:
- Lead is used as a colouring pigment in ceramic glazes, most commonly bright red and yellow and, historically, has also been found in other industries using pigment based finishing systems, such as the leather industry.
- Lead is a major constituent of the lead-acid battery used extensively in cars.
- Lead is used as a projectile for firearms and fishing sinkers because of its density, low cost compared to alternative products and ease of use due to its relatively low melting point.
- Lead or ‘sheet-lead' is used as a sound-deadening layer in such areas as wall, floor and ceiling design in sound studios where levels of airborne and mechanically produced sound are targeted for reduction or virtual elimination.
- Lead is used in some candles to treat the wick to ensure a longer, more even burn. Because of the dangers, European and North American manufacturers use more expensive alternatives such as zinc.
- Lead is used as shielding from radiation.
