Many types of plastic are given a recycle code along with an abbreviation for the plastic type. For example #1 is PET or PETE (Polyethylene Terephthalate). The least toxic planters are labeled as #2 HDPE, #4 LDPE, and #5 PP. The unsafe plastics include #3 PVC, #6 PS, and #7 polycarbonate.
However, any chemical is safe provided that we are not exposed to high levels of it, and the level is different for every chemical.
Watering also has a big effect on how much of the chemical stays in soil. Containers are watered frequently and it is common to water until it runs out the bottom. This helps wash away any chemicals, including fertilizer. Another process that takes place in soil is the absorption of chemicals by organic matter. A higher organic level results in less of the chemical being available to roots.
What all this means is that much of the chemicals leaching from plastic never make it to the roots.