Parts per million (ppm) is a unit expressing the concentration of one substance within another as the number of parts of solute per million parts of solution or mixture. It is the standard language for trace levels in food safety and nutrition: pesticide residues, heavy metals like lead and cadmium, mycotoxins, and water contaminants are all regulated in ppm or its smaller cousin, parts per billion. One ppm equals one milligram per kilogram or one milligram per litre in water, a conversion every food scientist memorises. The unit makes vanishingly small quantities graspable: a single drop of ink in a large bathtub is roughly one ppm. Regulatory limits set in ppm reflect toxicological risk assessments, and exceeding them can trigger rejections, recalls, or legal action, making accurate ppm measurement the bedrock of contaminant control.