Is your garden soil safe to grow food in?

Where the contamination comes from

Lead is the most common contaminant in urban soil². In soils tested at six sites in Ohio neighborhoods, paint appeared to be the main source of lead, and the soils also held zinc, cadmium and copper¹⁰. In New Orleans, median topsoil lead fell 44% over 15 years while children's blood lead fell 64%, and the authors concluded that falling topsoil lead was an important factor in the decline¹¹. In Washington's Yakima Valley, orchards were sprayed with lead arsenate from the early 1900s to the 1950s, leaving lead and arsenic in the soil⁹, and older pressure-treated wood made with chromated copper arsenate (CCA) can leach arsenic⁸.

How people are actually exposed

In an exposure model for 141 backyard gardens in Boston, homegrown produce accounted for about 3% of a child's daily lead exposure and swallowed fine soil for 82%¹. A review reached a similar view: plant uptake of lead is typically very low, with exceptions in low-growing leafy crops, where soil splashes onto leaves, and in root vegetables such as carrots². A field survey found most lead concentrated in roots, fruiting vegetables below the detection limit, and some leafy vegetables and herbs high enough to add to body burden³.

What the guideline numbers mean

In January 2024 the US EPA lowered its residential soil lead screening level from 400 to 200 parts per million, or 100 where other lead sources exist. These are screening tools for flagging sites, not cleanup rules or a line between safe and unsafe, but they help you read a lab result⁵.

What reduces the risk

Compost and soil amendments diluted soil lead and cut lead in vegetables by 50-71% in field plots⁶. Raised beds help but can be recontaminated by wind-blown lead dust, so they need maintenance¹. Household washing removed about 56% of lead on leafy vegetables on average, though adhering soil particles can remain^4,12. Keeping soil out of the house is a standard practice².

For another lead source, see Lead in Tap Water. For a metal that arrives with fertilizer and crops, see Cadmium in Food. The benefits of growing food are covered in Is Gardening Good for Your Health?.

Lead in garden soil and crops

Clark and colleagues tested 141 backyard gardens in Roxbury and Dorchester, Massachusetts, by X-ray fluorescence and found 81% above 400 micrograms per gram¹. Raised beds started at 150 (plus or minus 40) micrograms per gram but averaged 336 four years later, and wind-transported particles with a lead-paint signature were the mechanism¹. Their exposure model attributed about 3% of children's daily lead exposure to homegrown produce and 82% to ingestion of fine soil (under 100 micrometers)¹. Limitations: a model with assumptions, and one neighborhood; the authors conclude remediation needs constant maintenance and may need to happen at neighborhood scale.

Finster and colleagues surveyed garden plants grown in contaminated residential soils, washing samples with detergent or water, and found lead mostly in roots (root-to-soil ratios 0.02-0.51), with shoot-to-soil ratios up to 0.10³. Fruiting vegetables were below the 10 ppm detection limit, while some leafy vegetables and herbs had levels that could add to body burden³.

Defoe and colleagues set up plots in Tacoma (soil lead 51-312 mg/kg, arsenic 39-146) and Seattle (lead 506-2,022, arsenic under 20) and tested a biosolids mix or compost plus dolomite⁶. Amendments diluted soil lead by 10-23% in Tacoma and 20-50% in Seattle, lowered vegetable lead by 50-71% and arsenic by 46-80%, and laboratory cleaning reduced the transfer of lead and arsenic from vegetables⁶. Limitations: two sites and three vegetables.

Washing and clinging particles

Augustsson and colleagues analyzed 63 samples of chard, kale, lettuce and parsley grown near a busy road in Sweden, with half of each sample washed⁴. Washing removed on average 56% of lead, 45% of arsenic, 13% of copper and 7% of cadmium, and without washing the average daily lead intake would be 130% higher⁴. A companion paper found that vegetables still carried 0.05-1.3% adhering particles by weight after washing; at 0.5%, an adult eating an average amount of vegetables could ingest about 100 mg of particles a day¹². Particles supplied 25-29% of vegetable lead, and across 20 urban soils from five European countries, lead posed the greatest risk, with hazard quotients often above 1 for particle ingestion in adults and 3-6 year-olds¹². These are models of hypothetical scenarios, but they suggest that limiting dust and soil splash matters as much as choosing clean soil.

Optimistic and cautious readings

Brown and colleagues concluded that it is highly unlikely that urban agriculture will raise children's blood lead, because gardening tends to improve soil, plant uptake is typically low and little lead is absorbed when eaten with food, and that the benefits of urban agriculture far outweigh the risks². The Swedish modeling is more cautious about clinging particles¹². The views are compatible: the open question is how much adhering soil matters, which varies by site.

Children and soil lead

Mielke and colleagues compared 274 New Orleans communities across about 15 years: median topsoil lead fell 44% (99 to 54 mg/kg) while children's blood lead fell 64% (3.6 to 1.2 micrograms per deciliter)¹¹. This is an ecological comparison, and other lead sources fell too. Lanphear and colleagues pooled seven cohorts of 1,333 children followed from infancy and found IQ fell by 3.9 points (95% CI 2.4-5.3) as blood lead rose from 2.4 to 10 micrograms per deciliter, with a steeper loss per unit at lower levels than at higher ones⁷.

Other sources: pesticides, treated wood, vacant lots

Durkee and colleagues sampled soil at 19 childcare centers in Washington's Yakima Valley, where orchards were sprayed with lead arsenate from the early 1900s to the 1950s⁹. Four (21%) exceeded the regulatory standard for lead and eight (42%) for arsenic, and historic land use was significantly associated with elevated levels⁹. A review of CCA-treated wood notes that it is restricted in most countries but still present in outdoor structures that leak metals, with children the main concern⁸. In soils from six locations in low-income Ohio neighborhoods, lead, zinc, cadmium and copper were elevated, paint was the main source, PAHs were well below levels of concern and no foodborne pathogen targets were detected; long-established gardens had lower contamination, perhaps because tillage mixed in cleaner soil¹⁰. Limitation: six sites.

Bottom line (confidence 3 of 5)

Lead in soil is a real, well-documented exposure, mainly through swallowed soil and dust, and testing is worth doing before you grow food in older urban or orchard soils. Crop uptake is usually low, but leafy and root crops and soil clinging to leaves can matter at some sites. The protective steps are simple and cheap: clean beds, compost, mulch, washing, and soil kept outside.