These particles in fuel can be road dust, engine rust, wear particles or any other hard particles. Particle contamination gets into diesel fuel in multiple ways. The fuel itself can pick up particles during transit. Particles can also ingress through the tank vent. As the fuel tank is drawn down, ambient air is drawn into the tank, providing a source of particle contamination. Wear debris from fuel system components is another source.
Today, particles significantly smaller than 4 μ in size are potential wear contributors to important parts.