The short answer is that urban planter boxes can be safe for growing edible greens, but they are not automatically safe just because they are sold as planters. Whether chemicals leach from the material depends on what the box is made from, how old it is, how acidic or alkaline the soil is, how much heat and moisture it experiences, and how the container is maintained. In many cases, the greater risk in urban food gardening comes not from the planter itself but from contaminated soil, dust, water, or unknown recycled materials.
Some materials are generally considered lower risk. Food-grade high-density polyethylene (HDPE), polypropylene, stainless steel, lead-free glazed ceramic, and untreated rot-resistant wood such as cedar are common choices for edible container gardening. New food-grade plastic is usually preferable to unknown recycled plastic, because recycled plastic can contain contaminants from its previous use. Untreated wood is a reasonable option, but it will eventually degrade; avoid old railway ties, utility poles, and pallets with unknown treatment histories.
Pressure-treated wood deserves caution. Older wood treated with CCA (chromium, copper, and arsenic) should not be used for edible gardens. Modern pressure-treated wood using ACQ or copper azole is less toxic than CCA, but it can still release copper and other compounds, especially in acidic soil or wet conditions. If you already have a treated-wood planter, line the inside with a food-grade liner, maintain drainage, and avoid direct contact between the soil and bare treated wood. Do not use treated-wood scraps unless you know exactly what they contain.
Galvanized metal can also be a concern in some conditions. Zinc can leach into soil, particularly when soil is acidic or when certain fertilizers are used. A food-safe liner can reduce contact. Painted containers should be treated with suspicion unless the paint is known to be lead-free and suitable for food gardening. Flaking paint, old paint, and unknown sealants should be removed or sealed away from the growing medium.
Leaching is real, but it is usually a matter of degree rather than a simple yes-or-no answer. Acidic soil, high heat, UV exposure, poor-quality plastic, aged materials, and constant moisture can increase the release of chemicals. Leafy greens such as lettuce, spinach, and kale can take up some heavy metals from contaminated soil, so using clean potting mix rather than unknown street soil is important. In urban areas, also consider lead dust, air pollution, and runoff. Place containers away from busy roads, wash greens thoroughly, and cover bare soil with mulch to reduce dust settling.
For the safest approach, use food-grade or clearly safe materials, avoid treated wood and unknown paints, line questionable containers, ensure drainage, and use fresh potting mix. If you are gardening on a balcony or rooftop, keep containers away from peeling paint and contaminated runoff. If you have concerns about an older or unusual container, a soil test can help. Overall, urban planter boxes can be a safe and productive way to grow edible greens when you choose materials carefully and manage the growing environment.