Australis said:There will almost certainly be mould spores around, but if your plants are healthy and get adequate ventilation and light, it won't be a problem. It is mainly when they stay too wet and cannot breath properly that problems with mould start to develop. If you see mould, remember that it's probably a symptom of an underlying problem, rather than a problem just by itself.
Australis said:How bad is your allergy? There's going to be mould spores around if you have houseplants, so if they generally don't bother you, it comes down to making sure the conditions don't favour the growth of mould. As I mentioned, ensuring plenty of light and airflow as well as not overwatering will minimise the risk of any mould growing.
Australis said:I probably wouldn't repot them at the moment, but it is up to you. You have to balance the probability of mould growing vs the stress to your plants by repotting them again.
Try to keep your houseplants where they will get good airflow and sunlight (you could even use a fan to help circulate air and dry out the mix). Let the potting mix dry out somewhat before you water them again; you don't want your plants to dehydrate, but the mix doesn't need to stay sodden, either. This will help discourage any mould growth.
A problem with using any of those (baking soda, vinegar, hydrogen peroxide) is that if you apply them to a pot plant, it can upset the balance of the soil and the plant will suffer (or possibly die). If mould does start to grow, then wear your mask and take the plant outside. The extent of the mould growth will determine the best course of action at that stage; often it is possible just to remove the affected bit of soil and allow the rest to dry out thoroughly.
WillC said:If a plant is in soil that is water-logged, it is best to determine why. Was there no drain hole? Is someone else watering your plant? Is the plant in a pot that is too big or in light that is inadequate? Simply replacing the soil with dry soil is not a very good solution and may cause a lot of other problems.
Bagged potting mixes often contain non-sterile ingredients such as bark chips, soil, and compost all of which may contain fungus spores and fungus gnat larvae. This is a potential problem even if the bag is fresh off the retailer's shelf. Look for a potting mix that contains only peat moss, coir, perlite, lime and some fertilizer. Unfortunately, you have to find the ingredients and they are in fine print on the bag. As an alternative, you can mix your own by using 4 part of peat moss and 1 part perlite.