Black mould is sometimes seen around window frames, external doorways and in places that often stay damp – like bathrooms. The cause is that moisture-rich air: as the humidity rises, the mould spores that are in the air all the time can settle out onto surfaces where the conditions are just right for them to thrive. It’s usually a problem that is more pronounced in Autumn and winter because we tend to close windows more, driving the humidity in the room upwards. In summer, when we keep windows open because the room temperature is higher, the fresh air from outside mixes with the room’s air and not only reduces the number of mould spores that are concentrated in the room, the humidity of the air is lower and that means the spores are less able to reproduce.

The current economic climate and rising cost of energy means more and more people are turning the heating down – and while this saves money, one downside is that it increases the risk of mould being able to thrive in rooms we live in. This black mould is a health hazard and linked to breathing difficulties – and worse, especially for people with existing health issues.

Getting rid of this mould is key. Using a damp cloth and wearing gloves (and a mask, ideally the kind of mask that we used for Covid), wipe off as much of the black residue as possible. If the cloth is reusable, rinse it well between each wipe to prevent spreading the problem – but disposable kitchen wipes that are thrown in the bin might be a good idea for this job.

Once the mould has been removed, we need to apply something to kill off the remaining spores. This is where Granny’s advice comes in: while there are perfectly good commercial products that claim to be ‘mould killers’, what we need to do is alter the local environment to make it unsuitable for mould – and the easiest way to do that is change the acidity levels. White vinegar will do this and it’s almost certainly the cheapest way, too! Dilute it with water to about half strength and dab it onto the mould with a disposable cloth. The job isn’t to soak the wall so badly that the liquid runs down the walls, just give it a good dabbing and let it dry. Return and do it again a few more times over the next few days and you should find mould growth is significantly slowed down.

Of course, we do need to do something about the damp problem that made conditions so perfect for the mould to form in the first place – so air the room (open a window) or, if the mould is caused by damp within the wall, do your best to get the root cause of that dampness resolved. That could be damage to the mastic sealing around a window, rotting wood in a doorframe or simply a leaking gutter that’s causing the brickwork to get soaked every time it rains – but getting to the root cause is vital. If the problem is only in a bathroom, you might find that wiping down the surfaces after having a shower so there’s no standing water will make a significant difference; less water in the room means less that will evaporate into the air and build the vapour levels up. In kitchens, keep a window open while cooking, keep lids on pots (economically sensible anyway – putting a lid on a pot means it needs less heat to keep boiling) and give surfaces a wipe down to remove surface water.

Using a dehumidifier to reduce the water vapour in the room may be an option for bedrooms or living rooms. They’re cheaper to run than putting the heating on but generally won’t work that efficiently if the room is below about 18oC: they rely on cooling and condensing the water vapour – and that’s not a process that is very effective when the air temperature is already low.