Are Thermal Labels Toxic? A Warning for Those Who Scratch Receipts
Whether it's at the supermarket checkout or ordering takeout, we often receive a receipt with clear black text on a white background. Many people simply throw it away after receiving it. However, those who pay attention will notice that these receipts have a magical "skill." They turn black when scratched with a fingernail, a method often used by restaurant waitstaff to record the dishes that have been served. What is the scientific principle behind this? Is it toxic to turn black with a scratch? What harm can it do to the human body?

Why Receipts Turn Black with a Scratch

The paper we commonly use for supermarket shopping receipts and movie tickets is called thermal paper. As the name suggests, it is very sensitive to temperature changes and turns black when heated. When we scratch it with our fingernail, the friction generates heat, causing the corresponding part of the paper to turn black instantly.

Thermal paper is different from ordinary paper. It has a special coating on its surface, which is a thermochromic layer containing special dyes. The key components are two types: one is a colorless dye (or called leuco dye), which is initially colorless; the other is a developer, which reacts with the colorless dye when heated, changing its structure to a colored form. When the machine prints, the high-temperature print head inside heats the thermal paper, causing the colorless dye molecules and the developer in the thermochromic layer to react and leave a mark, resulting in the black color. This is a groundbreaking invention that allows printing without ink by using these two substances to form a color-changing system.

Long-Term Contact Is Toxic, but Daily Use Is Not a Concern

Strictly speaking, the upper layer of thermal paper is coated with some chemical raw materials such as bisphenol A (BPA), which can be harmful to the human body with long-term exposure. The main concern is BPA, which can damage liver and kidney function and is an endocrine disruptor that may cause endocrine disorders. For pregnant women, BPA can be harmful to the fetus, potentially causing fetal deformities. For adults, BPA has a greater impact on men than on women, mainly affecting male fertility.

Note that this is only a concern with long-term contact! If you are a cashier, you should be more cautious. However, if you only come into contact with thermal paper occasionally at supermarkets or restaurants, the toxicity is negligible. Different thermal papers use different thermal coatings, so it is advisable to choose high-quality thermal paper to avoid the hazards of hazardous chemicals.

Why Does Thermal Paper Fade Over Time?

Thermal paper has a disadvantage in that the printed content will gradually disappear over time. To address this issue, people have improved it to create a new type of thermal paper called PTM, which has greatly improved image stability. In addition, there are now dual-color thermal papers available. The reason for this fading phenomenon is that the leuco dye in the thermal paper undergoes a chemical change at high temperatures, forming an unstable colored structure. Once exposed to air, it reacts with various chemicals, and these tiny chemical interactions can cause the dye to revert to its colorless state.

Can Faded Text Be Made Visible Again?

The answer is yes. The same method that makes the text disappear can be used to make it reappear, which is by heating. Continued heating will cause the white areas of the thermal paper that have not undergone a chemical reaction to turn black; the areas that already have text, having undergone the reaction, cannot react again, and will appear as white marks, making the text visible again. Although this method cannot guarantee the clarity of the text, and the duration of visibility will be greatly reduced, in many cases, the text can be clearly seen after heating. If necessary, take a photo immediately for future reference. Understanding this principle, you will know that this method is also a scientific and effective way to quickly erase personal information on courier slips. However, the entire process must be carried out with caution, avoiding open flames, and heating the back of the thermal paper evenly against the heating surface of the heating tool.