A hot empty wash cleans the machine

Does it work?

Unresolved

The contamination is measured and real; the remedy is simply untested. An absence of evidence is not a result in either direction.

This is a verdict on whether the technique is effective. Whether it is safe on a particular surface is a separate question, answered below.

The proposed mechanism

The explanation below is what is offered for this technique. No evidence establishes that this is what actually happens, so treat it as a plausible story rather than as a finding.

Heat and detergent circulating through an empty drum are supposed to kill and flush what has accumulated. The mechanism is plausible for surfaces the wash water actually scours, and the measured problem sites are the ones it scours least: a drawer chamber the flow passes through in pulses, and the underside of a seal fold. No study tests either the treatment or the temperature.

Background

The advice is everywhere and the underlying worry is justified. A peer-reviewed study sampled domestic washing machines and concluded that detergent drawers and bottom door seals are significantly contaminated with cultivable bacteria, including a significant share of potentially pathogenic ones. So there is something there. What the study did not do is test any intervention, and it did not examine wash temperature, so it supports no claim whatsoever about whether a hot empty cycle helps. That cannot be supplied by inference, and the distribution of the contamination is the reason for the caution rather than mere pedantry. The two worst sites were the detergent drawer chamber and the bottom of the door seal — the two places a cycle's water flow reaches least well and which stay wet longest afterwards. A hot cycle passes water through the drum, and a seal's lower fold and the chamber behind a drawer are precisely the geometry a flow of water does not scour. What the measurement does point at is drying: the top of a door seal, which drains, was orders of magnitude cleaner than the bottom of the same seal.

What it does not do

• The study quantified contamination and evaluated no treatment of any kind
• The study did not examine wash temperature, so it supports no claim about hot cycles
• The two most contaminated sites are the ones a cycle's water flow reaches least