Lead is still poisoning urban soils. Photo: subpng.com, uokpl.rs & onlygfx.com

I had a horrible scare back in the mid-1980s. A news report came out about children in my neighborhood being poisoned by eating lead paint and the report further suggested that vegetables grown near old painted walls could be contaminated with lead.

Of course, where was I growing my vegetables, but right at the base of a very old wall indeed, one dating back to the 1850s, with plenty of ancient paint flaking off? I think I drove my city counselor half crazy with calls insisting that my 4-year-old son and the soil of my garden be tested right away! Well, both were: my son, right away; the soil, eventually. My son was fine (we had only just moved in and had only harvested the very earliest spring vegetables). But soil was indeed contaminated. Over 4,000 ppm of lead, if I remember correctly. I was told to not grow root vegetables there are all, that leafy vegetables needed to be thoroughly cleaned and rinsed, but that fruiting vegetables would be fine. (Those were the recommendations back in the 1980s.)

However, fat chance I’d take the risk of growing anything edible in that lead-tainted soil! I had a child to think of and children are much more susceptible to lead poisoning than adults. I had already pulled out all that first year’s crop after the initial report and put them straight into the trash. I didn’t even dare put them in the compost bin! And I started vegetable gardening in a community garden, far from any paint, the following spring. 

The old vegetable garden became a flower bed for the next few years, until we moved into a newer development in suburbia where we still live. Although, due to its lesser age, our current home is not as likely to have been contaminated with lead-based paint, I still put the vegetable garden far from the house … and there it remains. Once burned, twice shy!

So, the original incident happened nearly 40 years ago. Lead paint and leaded gas were banned in most developed countries somewhere between the mid-1970s and the mid-1980s.

What’s the situation today?

What follows is an article by Anna Wade of Duke University from the excellent site, Soils Matter, Get the Scoop! of the Soil Science Society of America, a go-to source for valuable and honest information on the soils we garden in.

Is lead contamination ancient history?

Lead’s use may be—but we still have a lead problem, especially in urban areas.

The first extraction of the metal lead from ores was ancient—around 7,000 BCE. In the millennia since, Egyptians have used it in cosmetics, Romans in their pipes, the British in their ammunition and now every society in lead-acid batteries.

And, if it were not so toxic to humans, the use of lead would still be widespread in our daily lives.

Lead is a “heavy” metal, meaning it’s a dense element. Lead is also soft, malleable, corrosion resistant and distinguished by a low melting point. That’s what gives it its useful characteristics.

Yet lead is a highly poisonous metal. Its presence disrupts almost every organ in the body if inhaled or swallowed. Lead displaces other metals in the body, such as calcium and iron, disrupting chemical reactions. The most problematic effects are on children. By mimicking calcium, lead can enter a child’s developing brain and disrupt the functioning of mitochondria.

Currently, there is no known “safe” level of blood lead concentration in children. Since 1960, the Center for Disease Control’s advisory level for blood lead has dropped from 60 micrograms per deciliter to 5 micrograms per deciliter.

In the United States, risk of lead exposure is becoming more of a thing of the past. In the 1970s, lead-based paint was common, as it increased the paints’ durability and sped drying. It was also used in “leaded” gasoline, which made car engines of that generation (and prior) work more smoothly.

Unfortunately, the lead from gasoline was also sent into the atmosphere through car exhaust. It landed in the soil everywhere. The higher the concentration of cars—like cities and highways—the more lead.

Lead paint is a big risk for children. Medical studies showed lead exposure affected learning abilities and other health issues—especially in children. Lead paint that peeled off—or created dust—could be ingested or inhaled. This is especially true close to exteriors of older homes, around windows and other locations where lead paint was used.

Luckily, the U.S. banned manufacture of lead-based paint in the late 1970s. The EPA mandated a nearly 100% reduction of lead in gasoline by 1986. Between 1980 and 1991, mean blood lead levels of children ages 1 to 5 dropped by 77%.

Despite the phase out of lead in paint and gasoline, urban soils remain recognized as a leading source of lead exposure. That also goes back to lead being a heavy metal. That character means it tends to accumulate in soils and remains bioavailable for long periods of time.

Once deposited, lead remains strongly bound to clay and organic matter in the topsoil. Lead is not taken up in substantial amounts by plants, nor does it easily leach or migrate further down in the soil. Instead, this lead remains as part of the reservoir of urban soil and dust, susceptible to resuspension during dry periods. This resuspension is why children’s blood lead levels are believed to peak during the summers and reach a minimum in the winter.

There is some good news. A recent study in New Orleans reported an approximate 45% decline in soil lead over the span of 15 years.

Currently, a group of students and faculty at Duke University are mapping soil lead concentrations in Durham, North Carolina. We’ve collected street-side soils along 40 km of roadways in the city, and we’re currently sampling over 60 houses throughout the city. The results show there is still widespread lead contamination from before regulations went into effect.

Your average soil will naturally contain about 25 parts per million (ppm) of lead. Street-side soils in Durham currently have 245 ppm of lead on average. Sites that are across from old gas stations, fire hydrants and older buildings have lead levels up to 3,000 ppm. Our only point of comparison is a 1976 study, which found 2850 ppm of lead in street-side soils.

This means that in some parts of Durham, lead levels may have dropped 90% from their peak values. Our results suggest urban soil lead is on average declining in Durham, but hotspots of contamination persist.

While the primary sources of lead emissions are in the past—leaded gasoline and lead-based paint—urban soil lead contamination is not. By mapping urban soil lead levels, we have a greater chance of making childhood lead exposure become ancient history.

Are You Concerned About Possible Lead Contamination in Your Garden? 

If you live in an urban core or garden at the base of an old building with painted walls, maybe you should be! Here are some things you could do*: 

1. Don’t locate food gardens next to a busy road or a home built prior to 1940 with a painted wooden exterior.
2. Contaminated soil particles are more likely to cling to or become embedded in leafy greens (lettuce, spinach) and root crops (carrot, turnip) and they are therefore likely to contain high lead levels than fruiting vegetables like tomatoes and cucumbers.
3. Always wash all vegetables and peel all root crops before they are cooked and eaten. Remove the outer wrapper leaves of cabbage.
4. Wash off excess soil from root and leaf crops outside the house, preferably at an outside hose bib, to prevent bringing contaminated soil into the home.
5. The amount of lead absorbed by plants is affected by the soil pH, organic matter and phosphorus content of the soil, and total soil lead level. To reduce lead uptake by plants, adjust the pH of the soil to a level of 6.5 to 7.0. Add organic matter such as compost, manure, leaf mold, or grass clippings to the gardening site. Add phosphorus to the soil as recommended by a soil test.
6. In heavily contaminated soils adjacent to a residence, don’t grow vegetables. Instead, plant trees, shrubs or perennials and mulch the area to keep the soil covered. Soil removal and replacement should be considered if the soil lead level is over 5000 ppm total lead.
7. Food crops should not be grown in soil that is over 400 ppm total lead. Use containers for gardening or cover the soil with landscape fabric and fill at least 8 inch (20 cm) high raised beds with a mixture of clean topsoil (low in lead) and compost.
8. Don’t allow young children to play in contaminated soils. Frequent hand washing and rinsing outside toys will reduce the amount of soil ingested. Always wash hands before eating meals or snacks. Have family members leave outdoor shoes in a cardboard box at the door, to avoid spreading lead contaminated dust through the home. Rinse and launder gardening clothing promptly. Mulch play areas with wood chips or other soft materials to reduce soil dust.
9. Parents of children under age 6 living in areas with contaminated soils should consult their physician. A blood test to monitor lead levels may be recommended.

*Information obtained from the website of the University of Maryland Extension.