Quote of the Day
Mythology is what we call someone else's religion.
- Joseph Campbell
Introduction
I saw all sorts of dangerous practices with lead as a kid. Here are a few examples:
- Fisherman who used their teeth to bite down on their sinkers
- Assemblers on production lines using their lips to hold solder.
- Eating game shot with lead projectiles (another reference)
- People sanding lead paint in their homes.
I shudder to think of the Romans using lead acetate as a food sweetener. Our use of lead as a gasoline additive, which put lead into the air, was probably not very health either. I saw a blog post where a construction worker was asking about how much lead there is in one square foot of painted wall surface. I thought it would be worthwhile to work this number out.
Background
History of Lead as a Pigment
The Wikipedia has an excellent description of the history of lead's use as a pigment. I quote the Wikipedia directly here:
Lead white was being produced during the 4th century BC; the process is described by Pliny the Elder, Vitruvius and the ancient Greek author Theophrastus.
The traditional method making the pigment was called the stack process. Hundreds or thousands of earthenware pots containing vinegar and lead were embedded in a layer of either tan bark or cow feces. The pots were designed so that the vinegar and lead were in separate compartments, but the lead was in contact with the vapor of the vinegar. The lead was usually coiled into a spiral, and placed on a ledge inside the pot. The pot was loosely covered with a grid of lead, which allowed the carbon dioxide formed by the fermentation of the tan bark or the dung to circulate in the pot. Each layer of pots was covered by a new layer of tan, then another layer of pots. the heat created by the fermentation, acetic acid vapor and carbon dioxide within the stack did their work, and within a month the lead coils were covered with a crust of white lead. This crust was separated from the lead, washed and ground for pigment. This was an extremely dangerous process for the workmen in the process; Medieval texts warned of the danger of "apoplexy, epilepsy, and paralysis" from working with lead white.
Despite the risks, the pigment was very popular with artists because of its density and opacity; a small amount could cover a large surface. It was widely used by artists until the 19th century, when it was replaced by zinc white and titanium white. In modern times we rarely use even those ingredients, with many paintings, wallpaper paints, and wall-art pieces using plastic-based paints made from acrylic (Check it out at Bumblejax for some examples). As you can see, we've progressed far from the need for lead-based paint, though it's hard to not imagine the maths behind the amount of lead in paint. How much would it take to poison a person?
Amount of Lead in a Can of Old Paint (Pre-1950)
The amount of lead in some cans of paint was as much as 50% by weight. Here is a quote that gives the range of lead content (Source).
Housing age is an important predictor of risk, because the lead content of paint varied substantially over the past century. While there is no clear dividing line, 1950 is often recognized as a threshold to lower levels of lead in paint. Prior to about 1940, paint typically contained high amounts of lead – often 10 percent and sometimes as high as 50 percent. In the early 1950s, voluntary paint industry standards called for limiting lead content to 1 percent, and in 1978 federal regulations effectively banned lead in residential paint.
Analysis
Figure 1 shows my analysis of:
- Amount of lead in a square foot of old lead paint.
- Amount of lead in the blood that causes serious illness
I do not know how readily lead dust gets into the blood stream, but I do want to compare the amount of lead available in paint to the amount you need in the blood to cause health problems.
Figure 1: Amount of Lead in a Single Square Foot of Paint.
Conclusion
This means that 10 square feet of painted surface can contain ~1 oz of lead. No wonder people worry so much about lead dust and the children eating paint chips (I have read that paint chips taste sweet). It is remarkable the amount of lead that must be in some older homes.