Increasing recycling rates requires not only better technologies but also smarter collection of plastic packaging waste1. Source separation—sorting materials such as plastics and metals at the household level—captures substantial waste volumes2,3, yet significant quantities still remain in the residual household waste fraction owing to misthrows and non-participation4,5. Post-sorting of mixed waste has been proposed as a one-bin alternative to boost capture6,7, despite concerns that contamination could compromise recycling quality7–9. Here we show, based on samples collected from one single material recovery facility, that bale purity, expressed as percent target polymer, is similar across source-separation and post-sorting pathways, but post-sorted bales contain more contaminants, including prohibited metals such as cadmium and lead. Post-sorted samples have higher moisture and dirt content8, which can lead to increased complex volatile organic compounds and necessitate additional washing. Concentrations of metals and halogens are elevated owing to non-packaging items, potentially compromising recycling quality and further complicating both mechanical and chemical recycling processes10,11. Although post-sorting can be a useful supplement, it should not replace source separation. Our results demonstrate that post-sorting can increase feedstock for recycling, but it also acts as a pathway for certain contaminants to enter plastic packaging waste, raising concentrations above typical levels, with potential risks to human health if these contaminants are not removed before recyclate production. As post-sorting of residual waste becomes crucial to meet circularity targets, these findings are particularly relevant.
