Leaving frozen dumplings in a car for 3.5 hours can potentially lead to a loss of quality and raise safety concerns regarding foodborne illnesses. The general rule for perishable foods, including frozen items, is the "2-hour rule." Perishable foods should not be left at room temperature for more than 2 hours (or 1 hour if the ambient temperature is above 90°F or 32°C). In this case, the dumplings were left in the car for 3.5 hours, which exceeds the recommended time frame.
The main risk associated with leaving frozen dumplings at room temperature for an extended period is the potential growth of harmful bacteria as they thaw and reach temperatures within the "danger zone" (between 40°F or 4°C to 140°F or 60°C). Within this temperature range, bacteria can multiply rapidly and pose a health risk if consumed.
As a safety precaution, it is best to discard the dumplings to avoid any risk of foodborne illness. Even if you have returned them to the freezer, the time spent at room temperature may have allowed bacteria to start growing, and refreezing will not eliminate any potential toxins or spoilage.
To maintain food safety in the future, always be mindful of the 2-hour rule for perishable foods. If you plan to transport frozen items, use a cooler with ice packs to keep them at a safe temperature during transit. When in doubt about the safety of any food item, it is better to err on the side of caution and avoid consuming it.