1. Ingestion of lead compounds trapped in the upper respiratory tract or introduced into the mouth on fingers, food, tobacco, or other objects.

“Lead occurs in water in either dissolved or particulate form and is more easily absorbed at low pH. Significant quantities of lead can be ingested from stagnant water in pipes or water coolers with lead solder. The popularity of home renovation poses a serious health hazard from the paints containing lead pigments (used in Australian houses prior to 1970). Cases of paint ingestion have increased markedly. Children are apparently attracted to the paint because of the sweet taste of lead acetate found in these paints. Research by CSIRO [4] has confirmed that leaded paint particles invisible to the naked eye can be transported some distance and contaminate the wider neighbourhood. Houses previously ‘de-leaded’ were found to have been contaminated by lead paint from other renovations or from deteriorating surfaces in the neighbourhood.”

“A large amount of the ingested lead passes through the body unabsorbed and is eliminated in the faeces (adults absorb ~5-15% of ingested lead and retain less than 5%; children absorb ~50% and retain ~30%). Most of the absorbed lead is captured by the liver and partly excreted in the bile. Consequently, a large amount of lead is necessary to cause toxic effects by this route and a long period of exposure is usually necessary to produce symptoms.”

2. Inhalation of dust, fumes, mists or vapours.

“Common air contaminants are absorbed easily from the respiratory tract and symptoms tend to develop more quickly. If the lead particles are small enough to reach the alveoli of the lungs, then up to 70% of the inhaled dose can be absorbed. In industry (e.g., smelting, battery production etc.), inhalation is more common than ingestion. The Australian National OHS Commission (NOHSC) has recommended a standard for inorganic dust (such as PbO, PbO2 and PbSO4) of 0.15 mg/m3 air for TLA-TWA (Threshold Limit Value-Time Weighted Average).”

3. Through the skin.

“This route is most common in the case of organic compounds of lead such as tetraethyl lead (Tetra-ethyl lead, abbreviated TEL, is an organometallic compound with the formula (CH3CH2)4Pb. Once a common anti-knock additive in gasoline (petrol), TEL usage was largely discontinued because of the toxicity of lead. It is still used as an additive in the aviation fuel known as avgas.but it is not important for inorganic forms of lead.)

For the general population, exposure to lead occurs from inhaled air, dust of various types, food and water, with an almost equal split between inhalation and ingestion pathways. In the body, there is an active reservoir of lead (up to 10%) in the soft tissue and in the circulating blood (this is why a blood test is a good indicator of lead levels). The other 90% resides in bones where it replaces calcium but is innocuous. However, it may be remobilized under conditions where the body increases its demand for calcium, such as in times of high fever. Lead interacts with the thiol (-SH) groups in enzyme proteins and interferes with their chemical reactions. In most cases, lead accumulation in the body is reversible. This is clearly shown by the variation of blood lead levels during and after periods of workplace lead exposure at the Division.”

The following information has been taken directly from: http://www.minerals.csiro.au/safety/lead.htm