Do I really need an outdoor sample?
Ever wonder why mould inspectors take an outdoor sample? It might just seem like an extra cost, but honestly, it’s crucial.
An outdoor sample acts as a baseline so we can see what’s “normal” for mould spores in the air in your area in terms of the amount/type/composition. Indoor air should have a similar composition of mould to outdoor air, but in LOWER numbers.
If the outdoor spore count is 200 spores/m³ and your bedroom has 600 spores/m³, that’s a red flag. But comparing COMPOSITION of the indoor and outdoor samples is important too – and all too commonly we see inspectors simply declaring that “Yep, there’s fewer mould spores inside than outside so there’s no problem here”. Ahhh…. it’s not quite that simple.
If 20% of an outdoor spore sample is Cladosporium, but in your room Cladosporium accounts for 80% of the sample, something’s up.
If there are no Alternaria spores in the outdoor sample, but there’s 300 spores in your lounge room, that tells us something’s off. Plus the species present is important, inspectors should know which moulds are harmless indoors and which ones are toxigenic in any amount.
If the outdoor count is 200 spores/m³ and your kitchen has only 100 but 20 of those spores are Stachybotrys, that’s a serious problem.
There’s more nuance to interpreting mould samples than is relevant for this post, but know that a good inspector will not only rely on comparing indoor to outdoor mould counts – they will also use the control method to compare different rooms in your home to find any anomalies in the data that might suggest the presence of hidden mould. Imagine two bathrooms with totally different mould profiles—that’s a sign of a potential issue.
So, while outdoor samples might seem like just another charge, they’re actually key to a proper inspection. My first inspector skipped it, and I thought he was being nice and saving me money (until I realised I’d have to pay for a whole other inspection with someone else to have all the sampling done properly with an outdoor control).