A lawnmower can blow white smoke because of the natural workings of a petrol engine. Alternatively, it could be because of the owner failing to clean the machine adequately after use. A lawnmower can appear to be blowing white smoke because of the geology and climate of the UK. Component burnout is also a possible explanation.
If you have a petrol lawnmower, the white smoke may be photochemical smog. According to Honda, the combustion processes in a lawnmower engine cause emissions of hydrocarbons and oxides of nitrogen. In sunlight, these chemicals can react to form a photochemical smog. Some lawnmower manufacturers -- including Honda -- use a combination of lean carburettor settings and other protection systems to reduce the risk of photochemical smog.
Grass clippings combustion
After completing your mowing, you should clean the grass clippings from under the deck of your lawnmower. Usually, the manufacturer supplies a spanner/scraper tool for this purpose. According to Flymo, you should further use a soft brush to ensure no clippings remain in the air intakes. These could become a potential fire hazard. If you have failed to clean your lawnmower rigorously, grass clipping combustion could explain the white smoke.
If your lawnmower seems to be about to burst into flames because of either of the above or suspected component burnout, release the start/stop lever immediately. Disconnect your lawnmower from the Residual Current Device (RCD). Move away from your lawnmower if you are experiencing breathing difficulties. If your lawnmower begins to burn, use a fire extinguisher, if you have one. However, the Fire Service say you should only use a fire extinguisher where you feel it is safe to do so. Otherwise, telephone the Fire Service.
The “white smoke” you see might not be white smoke after all. It might be ground chalk powder. Many parts of the UK have chalky soil, according to The Royal Horticultural Society. Further, most of the country experiences winds of several knots at some time, as the Met Office’s UK wind map shows. If the rotating blade of your lawnmower strikes a piece of chalk, it can give an impression of white smoke, especially if chalk particles become airborne because of current wind conditions.