Active or inactive?

Mould exists in one of three states – it is actively growing, inactive, or non-viable (dead). While it is understood that active mould is hazardous to both the collection and those accessing it, it is also important to be aware that dormant and non-viable mould also present a risk to health (Fig. 1). The effects of exposure to mould are cumulative and around 2% of the population may develop an adverse reaction to it.

A diagram showing the three states of mould: active, dormant, and non-viable. Arrows between them show how active mould can become dormant mould, which in turn can become non-viable, but also how only dormant mould can return to being active. Above this is a double-headed arrow which spans the width of the diagram - it is labelled 'risk to collection', and ranges from very dark red to yellow as it follows the three stages from active (highest risk) to non-viable (least risk). There is another double-headed arrow beneath the three stages, which relates to health and safety risk. This arrow is completely orange, denoting that the risk is the same no matter the state of the mould.

Figure 1, Mould lifecycle stages and risk (produced by The National Archives)

When growing and reproducing, mould requires an organic food source, like paper, parchment, adhesive and other materials found in archival collections. It also needs moisture and thrives when the surrounding environment has a sustained relative humidity of +65%. When such conditions occur, germination and growth begin. Microscopic spores – or conidia – are released into the air and as they land on a surface that can support growth, they form chains called hyphae. These thread-like chains release digestive enzymes which degrade materials and subsequently absorb nutrients from it. These chains form the visible body of the fungus, known as a mycelium.

A diagram showing a cyclical spore process. Four stages are each connected by clockwise arrows - at the top is 'hyphal growth', then 'spore production', then 'spores', followed by 'spore germination', which returns to 'hyphal growth'.

If disturbed by air movement or by physical contact, spores are released into the surrounding area and, given the right environmental conditions, will go on to form further mould colonies.

When active mould growth is present, you may notice:

  • A perceptible musty odour, both within a storage space and on a document itself
  • A damp feel to the document surface
  • Noticeable changes to the appearance and condition of the document
  • Fluffy or slimy growths, which may smear when touched

Identifying and responding to an active mould infestation is essential. When left untreated, active mould can spread quickly, posing a risk to the ongoing condition of your wider collection, as well as to those who access it.

As humidity levels decrease, active mould becomes dormant and it:

  • Desiccates, becoming dry, powdery, or crusty and – like active mould – can be disturbed by movement
  • Can remain inactive for many years but may reactivate once humidity levels create a suitable environment
  • Can appear as mottling or staining on the document surface, often with dry, powdery deposits in the box – though appearance can vary widely

Because you cannot visually assess the difference between dormant and dead mould, (Personal Protective Equipment (PPE) should always be worn while handling affected collection material.

The science of mould has additional information on how mould grows.