Basically, you don't need to worry about unhealthy pollutants in picnic blankets, as long as you pay attention to labels, test seals and ingredients. For all reputable manufacturers, the information should be below the guideline values.
To be on the safe side, you can request the necessary information directly from the manufacturer, especially for unknown or foreign products.
Especially if you want to use the blanket for children or indoors, it should be guaranteed that it is free of harmful substances.