IEC 60721-2-7:2018

IEC 60721-2-7 ®
Edition 2.0 2018-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of environmental conditions –
Part 2-7: Environmental conditions appearing in nature – Fauna and flora

Classification des conditions d’environnement –
Partie 2-7: Conditions d’environnement présentes dans la nature –Faune et flore

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
either IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC
copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or
your local IEC member National Committee for further information.

Droits de reproduction réservés. Sauf indication contraire, aucune partie de cette publication ne peut être reproduite
ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie
et les microfilms, sans l'accord écrit de l'IEC ou du Comité national de l'IEC du pays du demandeur. Si vous avez des
questions sur le copyright de l'IEC ou si vous désirez obtenir des droits supplémentaires sur cette publication, utilisez
les coordonnées ci-après ou contactez le Comité national de l'IEC de votre pays de résidence.

IEC Central Office Tel.: +41 22 919 02 11
3, rue de Varembé info@iec.ch
CH-1211 Geneva 20 www.iec.ch
Switzerland
About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigenda or an amendment might have been published.

IEC Catalogue - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
The stand-alone application for consulting the entire The world's leading online dictionary of electronic and
bibliographical information on IEC International Standards, electrical terms containing 21 000 terms and definitions in
Technical Specifications, Technical Reports and other English and French, with equivalent terms in 16 additional
documents. Available for PC, Mac OS, Android Tablets and languages. Also known as the International Electrotechnical
iPad. Vocabulary (IEV) online.

IEC publications search - webstore.iec.ch/advsearchform IEC Glossary - std.iec.ch/glossary
The advanced search enables to find IEC publications by a 67 000 electrotechnical terminology entries in English and
variety of criteria (reference number, text, technical French extracted from the Terms and Definitions clause of
committee,…). It also gives information on projects, replaced IEC publications issued since 2002. Some entries have been
and withdrawn publications. collected from earlier publications of IEC TC 37, 77, 86 and

CISPR.
IEC Just Published - webstore.iec.ch/justpublished

Stay up to date on all new IEC publications. Just Published IEC Customer Service Centre - webstore.iec.ch/csc
details all new publications released. Available online and If you wish to give us your feedback on this publication or
also once a month by email. need further assistance, please contact the Customer Service
Centre: sales@iec.ch.
A propos de l'IEC
La Commission Electrotechnique Internationale (IEC) est la première organisation mondiale qui élabore et publie des
Normes internationales pour tout ce qui a trait à l'électricité, à l'électronique et aux technologies apparentées.

A propos des publications IEC
Le contenu technique des publications IEC est constamment revu. Veuillez vous assurer que vous possédez l’édition la
plus récente, un corrigendum ou amendement peut avoir été publié.

Catalogue IEC - webstore.iec.ch/catalogue Electropedia - www.electropedia.org
Application autonome pour consulter tous les renseignements
Le premier dictionnaire en ligne de termes électroniques et
bibliographiques sur les Normes internationales,
électriques. Il contient 21 000 termes et définitions en anglais
Spécifications techniques, Rapports techniques et autres
et en français, ainsi que les termes équivalents dans 16
documents de l'IEC. Disponible pour PC, Mac OS, tablettes
langues additionnelles. Egalement appelé Vocabulaire
Android et iPad.
Electrotechnique International (IEV) en ligne.

Recherche de publications IEC -
Glossaire IEC - std.iec.ch/glossary
webstore.iec.ch/advsearchform
67 000 entrées terminologiques électrotechniques, en anglais
La recherche avancée permet de trouver des publications IEC et en français, extraites des articles Termes et Définitions des
en utilisant différents critères (numéro de référence, texte, publications IEC parues depuis 2002. Plus certaines entrées
comité d’études,…). Elle donne aussi des informations sur les antérieures extraites des publications des CE 37, 77, 86 et
projets et les publications remplacées ou retirées. CISPR de l'IEC.

IEC Just Published - webstore.iec.ch/justpublished Service Clients - webstore.iec.ch/csc
Restez informé sur les nouvelles publications IEC. Just Si vous désirez nous donner des commentaires sur cette
Published détaille les nouvelles publications parues. publication ou si vous avez des questions contactez-nous:
Disponible en ligne et aussi une fois par mois par email. sales@iec.ch.

IEC 60721-2-7 ®
Edition 2.0 2018-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Classification of environmental conditions –

Part 2-7: Environmental conditions appearing in nature – Fauna and flora

Classification des conditions d’environnement –

Partie 2-7: Conditions d’environnement présentes dans la nature –Faune et flore

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 19.040 ISBN 978-2-8322-6246-7

– 2 – IEC 60721-2-7:2018 © IEC 2018
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 General . 5
5 Occurrence of fauna and flora . 6
5.1 Fungi . 6
5.1.1 Background . 6
5.1.2 Growth and survival factors . 6
5.1.3 Habitat and geographical distribution . 7
5.1.4 Effects of fungi on materials . 8
5.2 Bacteria . 11
5.2.1 Background . 11
5.2.2 Growth and survival factors . 11
5.2.3 Habitat . 12
5.2.4 Effects of bacteria on materials. 12
5.3 Insects . 13
5.3.1 Background . 13
5.3.2 Habitat . 14
5.3.3 Effects of insects on materials . 14
5.4 Rodents . 14
5.4.1 Background . 14
5.4.2 Effects of rodents on materials . 14
5.5 Algae and marine organisms . 15
5.5.1 Algae . 15
5.5.2 Borers . 15
5.5.3 Fouling organisms . 15
Bibliography . 16

Figure 1 – Map of regions with different degrees of fungal corrosion . 8

Table 1 – List of fungus resistant materials . 9
Table 2 – List of potential fungus nutrient materials . 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-7: Environmental conditions appearing in nature –
Fauna and flora
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and
non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates
closely with the International Organization for Standardization (ISO) in accordance with conditions determined
by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60721-2-7 has been prepared by IEC technical committee 104:
Environmental conditions, classification and methods of test.
This bilingual version (2018-11) corresponds to the monolingual English version, published in
2018-03.
This second edition cancels and replaces the first edition published in 1987. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) This edition has been entirely rewritten.

– 4 – IEC 60721-2-7:2018 © IEC 2018
The text of this International Standard is based on the following documents:
CDV Report on voting
104/741/CDV 104/792/RVC
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
The French version of this standard has not been voted upon.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60721 series, published under the general title Classification of
environmental conditions, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
CLASSIFICATION OF ENVIRONMENTAL CONDITIONS –

Part 2-7: Environmental conditions appearing in nature –
Fauna and flora
1 Scope
This document addresses the occurrence of fauna and flora, including its main effects on
electrotechnical products. Exposure and damage from the effects of fauna and flora can occur
at almost any time in a product's life cycle. Moreover, there are many agents of attack with
various actions.
This document addresses the occurrence and damage arising from fauna and flora in all
locations a product can be stored, transported or used. Generally, fauna can be present and
cause damage to products in both the natural environments experienced in open-air locations
as well as in artificially created environments, such as in a warehouse or building. However,
flora will predominantly be present and cause damage to products only in open-air locations.
Fungus and bacteria can be present in both open-air locations as well as in warehouses or
buildings.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 General
The main attacking agents considered in this document are micro-organisms including fungi,
bacteria, as well as insects, rodents, algae and marine organisms. Hazards due to other
agents are considered to be of lesser importance and have been omitted. These include the
corrosive action of juices secreted by some plants, the mechanical action due to the growth of
the larger trees, which may be sufficiently great to destroy the foundations of a building or to
break cables, and the damage caused by animals such as monkeys and elephants. Birds in
flight can be a hazard to aircraft, and in the region of bird colonies, widespread droppings can
create corrosion problems. In addition, some agents which are mentioned have other modes
of action which have not been included; for example both rodents and insects are occasionally
responsible for chemical corrosion or soiling.
The frequency of occurrence of fauna and flora with a possibility of damaging products very
much depends on conditions of temperature and humidity. In geographical areas with warm
damp climates, fauna and flora, especially insects and micro-organisms such as mould and
bacteria, will find favourable conditions of life. Moreover, humid or wet rooms in buildings, or
rooms for processes producing humidity, are suitable living spaces for rodents, insects and
micro-organisms.
– 6 – IEC 60721-2-7:2018 © IEC 2018
Fauna and flora can affect products in various ways, the most important of which are given in
the following examples.
a) Deterioration by physical attack: The functioning of products may be affected by physical
attacks of fauna and flora. The materials of a product may be attacked by fauna,
particularly by rodents and insects, by the actions of feeding from material, gnawing at
material, eating into material, chewing material or cutting holes into material. The severe
damage arising from the physical attack by termites is especially emphasized in this
respect. Among materials susceptible to attack are natural materials such as wood, paper,
leather, textiles, but also plastic materials, including elastomers and even some metals
such as tin and lead.
b) Deterioration by deposits: The functioning of products may be affected by deposits
originating from fauna and flora. These surface deposits affect the products by chemical
and mechanical reactions. Deposits from fauna, especially from insects, rodents, birds,
etc., may consist of elements such as the presence of the animal itself, the building of
nests or settlements, feed stock as well as the metabolic products such as excrements,
enzymes. Deposits from all kinds of flora may consist of material such as detached parts
of plants (leaves, blossom, seeds, fruits, etc.), growth layers of cultures of moulds or
bacteria and effects of their metabolic products.
5 Occurrence of fauna and flora
5.1 Fungi
5.1.1 Background
The name fungus is used to denote members of a large heterogeneous group of organisms, of
which there are about a hundred thousand known species. Most fungi are so small that they
can be observed only with the aid of a microscope. The terms 'mould' and 'mildew', although
not exactly defined in the biological sense, are used by both biologists and laymen to refer to
small non-parasitic fungi, such as those which do not live on other living organisms.
A fungus can, in general, be divided into two parts: the vegetative and the reproductive. The
vegetative part, known as the hypha, is essentially a threadlike filament normally having a
diameter between 2 μm and 20 μm and may be several centimetres long. In the simplest fungi
the hyphae are merely continuous tubes of living matter; in others they are divided by cell
walls, called septa, into separate cells. Collectively the hyphae are referred to as the
mycelium. The mycelium, together with the reproductive spores, is commonly observed on
mouldy bread, shoes, oranges, etc.
In the vast majority of cases the unit of reproduction is the spore. Normally it is unicellular and
microscopic, though occasionally, giants 500 μm in length occur. They may be produced
directly via the hyphae or from a structure created for this specific purpose, as in the
mushroom. From a functional viewpoint spores may be divided into two classes each of which
may be produced by the same organism: those which can be produced rapidly and in large
numbers but have little resistance to adverse environmental factors, and those which are
comparatively few in number but much more resistant to adverse conditions. The former
enable the fungus to spread rapidly during good growing conditions and the latter enable it to
survive hard times such as winter or drought and have been known to survive for many years
in a dry condition.
5.1.2 Growth and survival factors
In order to adapt themselves to changes in their environment or food supply, most species of
fungi can slightly change their characteristics and needs over several generations. This may
be a very short time; in many cases the whole cycle from spore to spore can be completed in
a few days. In addition, it should be noted that the conditions required for the production and
dispersal of spores are generally more exacting than those for growth and survival.

The precise minimum, maximum and optimum temperatures for growth appear to be a matter
of debate between the various authorities. This may be because these values vary from one
species to another. However, in general, the minimum is 2 °C to 5 °C, the maximum 40 °C to
50 °C and the optimum 22 °C to 27 °C. In addition, there are a few fungi that can grow at and
below 0 °C, and one species has been reported growing at a maximum of 62 °C. They are, of
course, capable of surviving even greater extremes in a quiescent state.
The optimum humidity for the growth of nearly all moulds is a relative humidity of 95 % to
100 %. If submerged in water, however, most fungi will not grow. Any reduction from this
optimum will mean a reduced growth rate and few species will grow in a relative humidity of
less than 70 %. Optimum growth conditions also occur in still air.
A suitable source of carbon that can be absorbed as food is essential to fungi for their growth.
Almost all naturally occurring carbon containing compounds, together with many synthetic
organic compounds of a similar structure can be used by fungi as a source of food. All fungi
can utilize an organic supply of nitrogen and a few can also use an inorganic source such as
ammonia. Nitrogen, other than as a gas, is essential for the growth of fungi.
Most fungi are strictly aerobic, that is they cannot grow in complete absence of free oxygen.
In the small number of cases where fungi grow in water, they always do so in a few
centimetres near the surface.
Other elements required for the growth of fungi include sulfur (as sulfate), potassium,
phosphorus (as phosphate) and magnesium. In some cases minute traces of iron, zinc,
manganese, molybdenum, or calcium are required, though in such small quantities that only in
a few fungi is there a clear picture of these requirements. Some fungi also require a supply of
certain vitamins for growth.
Ultra-violet is known to inhibit the growth of most fungi, although daylight normally has no
effect. In a very few instances daylight can influence growth and indeed can cause it to
increase. However, the production and dispersal of spores is dependent upon the presence of
light for many species.
Most fungi grow best in a slightly acid medium within the range pH 5 to pH 6,5. This varies
from one species to another, but few will grow at all below pH 3 or above pH 9.
5.1.3 Habitat and geographical distribution
Since fungi can survive adverse growth conditions in a quiescent state and can gradually
evolve to survive more extreme conditions, and since new species are still being identified, it
is not possible to define exactly the geographical areas in which fungi will grow. There are,
however, certain tendencies which are relevant.
Fungi of one sort or another are found in the soil, water and air over a large part of the earth's
surface, whilst others live on or upon both living and dead animals and plants. Those found in
the air do not grow there, but are in the form of spores. Most live in the soil and only about
2 % live in water; in both cases they grow in the few centimetres just below the surface.
The best conditions for most types of mould growth are in humid tropical areas, although
deterioration due to mould is not confined to the tropics. Equally serious damage can occur in
temperate regions, though not so rapidly, and at least one species of mould is often found in
the form of spores in the air over arctic regions.
Conditions favourable for mould growth may easily be created artificially inside a building or
equipment. Those which are parasitic upon particular animals or plants are among the few
which are restricted to geographical regions.

– 8 – IEC 60721-2-7:2018 © IEC 2018
The map in Figure 1 shows areas in which climatic conditions are most favourable for fungal
corrosion. It is based on an analysis of relative humidity and temperature data from
approximately two thousand meteorological stations throughout the world, as follows:
a) Region A – includes areas with at least one month a year in which the mean monthly
relative humidity is from 70 % to 75 % in the hours from 12:00 h (noon) to 14:00 h, and
with a mean monthly minimum temperature at the same time of not less than 15 °C.
b) Region B – includes areas where the equivalent relative humidity is from 75 % to 80 %,
again with the same temperature as Region A.
c) Region C – includes areas where the equivalent relative humidity is greater than 80 %,
again with the same temperature as Region A.
IEC
Figure 1 – Map of regions with different degrees of fungal corrosion
It should be noted that the above climatic conditions do not take account of other naturally
occurring factors mentioned earlier, such as air flow. It also does not cover cases where
favourable conditions may be artificially induced, inside buildings or containers for example.
Nevertheless, within these limits, it does provide a useful indication of the natural liability to
attack by micro-organisms.
5.1.4 Effects of fungi on materials
Unlike most plants, fungi contain no chlorophyll, the green colouring matter with which plants
utilize the sun's energy to manufacture their food from absorbed raw materials. Thus they
have to rely on the food in the substratum on which they grow. However, the structure of the
cell walls only allows them to absorb this food if it is in solution. To achieve this, the fungi
secrete enzymes via their hyphae. This substance converts the food into a soluble form which
can then be readily absorbed.
There are three ways in which fungi may cause damage. Each can occur independently, or in
association with one or both of the others:

1) A material may be a food for the fungi, in which case the material is gradually eaten away.
2) The waste products of fungi are excreted as juices, many of which are corrosive and
cause damage to the substrate on which the fungi are growing. Thus it is possible for
fungi to damage a material even though it is not a source of food. For example the minute
impurities in finger prints on glass have been known to support growths whose corrosive
waste products have etched the surface of the glass. In addition, the mould coating has
the effect of retaining moisture and retarding the drying-out process.
3) Fungus may hinder the efficient operation of equipment, even though it has not caused
damage to any material part. Examples of this are soiling in optical equipment, or the
formation of undesirable conducting paths in electrical equipment.
The preferred method for controlling fungus growth is by the selection of fungus inert
materials. Also acceptable is the treatment of potential fungus nutrient materials or by
hermetic sealing. Table 1 lists materials which have a known resistance to fungus growth,
whilst Table 2 lists those materials which are potential fungus nutrients.
Table 1 – List of fungus resistant materials
Acrylics Polycarbonate
Acrylonitile-styrene Polyester
Acrylonitile-vinyl-chloride copolymer Polyester-glass fibre laminate
Asbestos Polyethylene, high density (> 0,94)
Ceramics Polyethylene terephthalate
Chlorinated polyether Polyamide
Fluorinated ethylenepropylene copolymer Polymonochlorotrifluorethylene
Fluorocarbon Polypropylene
Glass Polystryrene
Metals Polysulfone
Mica Polytetrafluoroethylene
Plastic laminate: silicone glass fibre Polyvinylidene chloride
Plastic laminate: phenolic-nylon fibre Silicone resin
Diallyl phthalate Siloxane-polyolefin
Polyacrylonirile Siloxane-polystyrene
Polyamide
– 10 – IEC 60721-2-7:2018 © IEC 2018
Table 2 – List of potential fungus nutrient materials
ABS (acrylonitrile-butadiene-styrene) Polyethylene, low density (< 0,94)
Acetyl resins Polymethyl methacrylate
Cellulose acetate Polyurethane (ester types are particularly
susceptible)
Cellulose acetate butyrate
Epoxy-glass fibre laminates Polyricinoleates
Epoxy-resin Polyvinyl chloride
Lubricants Polyvinyl chloride-acetate
Melamine-formaldehyde Polyvinyl fluoride
Organic polysulphides Rubbers, natural and synthetic
Phenol-formaldehyde Urea-formaldehyde
Polydichlorostyrene
A number of materials have a known susceptibility to damage by fungal growth. A number of
these are set out below.
a) Wood: Wood in contact with the ground is particularly prone to decay by fungi. If,
however, it is kept off the ground in a dry, well ventilated place it is much more resistant,
and if the wood contains less than 20 % water it is not attacked at all. Resistance to attack
varies from one species to another, and heartwood is always less liable to attack than
sapwood. In use, wood is normally coated or impregnated in some manner. This may
modify its resistance to fungal attack. Many fungi cause very little mechanical damage to
the wood on which they live but are found to discolour it.
b) Paper and cardboard: Paper, cardboard, and similar products are all susceptible to attack.
The basic constituent, cellulose, is affected as well as other substances used during
manufacture such as starch, gelatine. The effects are generally revealed as patches of
surface discolouration, followed by complete disintegration of the paper. However, mould
growth occurs after moisture pick up, and its damaging effects are often considered to be
second to those of moisture.
c) Paints and varnishes: Paints and varnishes are made of a complex mixture of oils,
cellulose derivatives, solvents, plasticizers, thinners, etc., some of which may be
susceptible to attack. Almost all paints will support mould growth under favourable
conditions. A few are resistant, but others have been known to support mould even in cold
storage rooms. The liability of a paint or varnish to attack is dependent on the type of
substance and surface on which it has been placed, and upon climatic conditions, such as
sunlight, moisture.
d) Natural cellulose fibres: Natural cellulosic fibres such as cotton, sisal, hemp, flax and jute
are all highly susceptible to attack, although protein fibres such as wool and silk are not
quite so liable.
e) Synthetic fibres: Synthetic fibres show some variation in their resistance, but are generally
superior to natural cellulosic fibres. Fibre forms of cellulose acetate, regenerated protein,
polyamides, polyesters, polyacrylonitrile, polyvinylidene chloride, vinyl chlorideacrylonitrile
copolymers, vinyl chloride-vinyl acetate copolymers, and glass all have excellent
resistance or are inert to fungi. Fibres may have pigment added or be coated, and
synthetic fibres may have other additives which can support growth (see also item g)
below). In use, when these fibres are in the form of a cloth or cord, they can easily hold
moisture and impurities which will nourish moulds.
f) Leather: Tanned leather generally has hygroscopic substances on its surface, such as
dextrin, starch, glycerine or sugar, which is often used for dressing, and may support
mould growth. Similarly, greases, oils and emulsions used for lubricating leather may also
support mould growth. In general, chrome-tanned leather is more resistant than vegetable
tanned leather. Mould rarely directly attacks the leather to any significant extent, even

when a profuse growth is found on its surface. However, if leather is exposed to mould for
protracted periods, enzymes secreted by mould will tend to digest fats, sugars and
carbohydrates in the leather, releasing various organic acids. Over time, these acids may
react with vegetable tannins producing signs of physical deterioration including loss of
strength and surface cracking.
g) Plastic and rubber: Plastic and rubber materials are not subject to deterioration in quite
the same manner as other materials such as wood. They nearly always contain one or
more additives for example, a plasticizer to reduce the natural brittleness of the plastic,
resin to increase the rigidity, pigments for colouring, fillers, stabilizers, etc. Each one is
added for a specific effect on the final product and each has a different effect on the final
resistance against attack. Hence, to predict the resistance of a given plastic or rubber
requires knowledge not only of the material, but also of all its additives, which may
constitute as much as 50 % of the total. In certain circumstances moisture rather than
mould will contribute to the deterioration of a plastic material, and since it is not possible
to have mould growth without the presence of moisture, there is some controversy about
which factor contributes most. From investigations it is clear that a given plastic may not
support the growth which is found on it, but is merely holding impurities on its surface
which nourish the mould, without damaging the plastic itself.
h) Glass: Glass is not a source of food, but its surface is readily covered with moisture to
which organic particles will adhere. This favours mould growth which in turn soils and may
even etch the surface, thus affecting the efficient use of optical instruments. In addition,
Canada balsam which is often used to cement lenses together is also susceptible.
i) Other materials: Other organic materials likely to be attacked include most foodstuffs,
adhesives, glues, inks, etc. There are also reports of deterioration of some hydrocarbons,
especially jet fuels; in this case the fungi are found in association with bacteria, and the
precise manner of deterioration is not clear. The metabolic products of cladosporium
resinae, the 'kerosene fungus' that grows at the jet fuel/water interface, are corrosive to
aluminium alloy fuel tanks.
5.2 Bacteria
5.2.1 Background
Bacteria are found in a variety of shapes and forms. They are generally unicellular organisms,
between 0,5 μm and 10 μm in length and 0,3 μm to 2 μm in diameter. Some species of
bacteria can move and possess flexible hair-like projections which enable them to do so,
whilst others are carried along in the medium in which they grow. Protection is afforded in
some cases by a gelatinous sheath or capsule, but in others this is absent.
A mature cell may reproduce by simply dividing in two. Later these two may divide again and
so on at intervals as small as 15 min. Many bacteria can also produce spores. However,
unlike the fungi, these spores are produced inside the bacterial cell, with normally one spore
per cell. These spores are more resistant to heat and drying than ordinary cells, and they
have been known to survive for many years in a dry state.
5.2.2 Growth and survival factors
As a consequence of their minute size, similarity of action to fungi and their association with
the same environments, some of the properties and actions of bacteria have not been isolated
or fully identified. In addition, bacteria can evolve and adapt themselves to new conditions.
Bacteria are less sensitive than fungi to temperature. The extreme growing temperatures are
a matter of debate but generally the range is from -3 °C to +75 °C. Like fungi, they can also
survive even greater extremes in the quiescent state. Optimum growth temperatures are
generally higher than for fungi. They normally require the presence of water for growth and
can flourish even when submerged in water. Consequently bacteria are found growing in
conditions that are wetter than for most fungi.
Many bacteria can make use of carbon only when supplied in an organic form. However,
others can grow in a medium devoid of organic material and make use of carbon dioxide as

– 12 – IEC 60721-2-7:2018 © IEC 2018
their source of carbon. Various bacteria can utilize organic and/or inorganic sources of
nitrogen. Some bacteria are aerobic, others are anaerobic and will grow only in the absence
of oxygen, whilst others can live either with or without oxygen.
Some vitamins and associated compounds have proved necessary to all bacteria, whilst
others are found to be required by only one or two species. Those bacteria which can grow in
media devoid of any organic materials obtain their energy requirements from the oxidation of
inorganic compounds such as ammonia, various compounds of sulfur and some compounds of
iron.
Most bacteria are active in the range of pH 6 to pH 8, but they can survive both higher and
lower pH values.
5.2.3 Habitat
Bacteria occur in soil, in natural bodies of water, in the air and internally as causal agents of
diseases in animals and plants. Most live in the soil, generally in wetter and slightly more
alkaline conditions than fungi, and are capable of surviving greater extremes of environment.
It is not possible to define exactly in which geographic areas bacteria will grow, and only
those which are parasitic upon particular plants or animals are at all restricted geographically.
5.2.4 Effects of bacteria on materials
Like fungi, bacteria possess no chlorophyll and secrete enzymes to digest food into a soluble
state before absorbing it through their cell walls. Also like fungi, they can evolve and adapt
themselves to new conditions.
Bacteria are generally too small to impair the operation of equipment. They can, however,
utilize a material directly for food and thus damage it. In addition, they can cause corrosion
when obtaining their energy from chemical reaction or by their waste products. They mostly
live in the soil near the surface, and it is here that they can cause most damage. Even so, it is
generally considered that in most cases more damage is caused by fungi than by bacteria.
Wood, leather, paper products and most associated materials, for example finishes, glues, are
all liable to bacterial attack in varying degrees. Natural fibres, of both animal and plant origin
are attacked by bacteria. The effect on synthetic fibres is not clear, although it appears to be
similar to that due to fungal attack, that is, viscose rayon, cellulose nitrate and a few others
are prone, but generally they are rethiobasistant. Most paints and varnishes are attacked by
bacteria and so are some rubbers and plastics. However, deterioration of these latter
generally occurs only in some specialized circumstances, as when permanently submerged in
water. As with fungi, it is often one of the many additives which are attacked, rather than the
basic material itself. Petroleum fuels and insulating oils are among the hydrocarbons which
are deteriorated by bacteria.
Bacterial attack is generally first noticed as a discolouration of the material concerned. In
some instances, however, such as insulating oils and the rubber-covering of cables, low
insulation resistance is often the first indication.
Bacterial corrosion is the term used to describe corrosion due to bacteria obtaining their
energy requirements from chemical action and to bacterial waste products. In these cases the
materials are not used as food. Ferrous metals are most liable to this method of corrosion, but
there are examples of attack on stone, rubber and non-ferrous metals. The organisms induce
a chemical reaction, for example oxidation, and use the chemical energy thus liberated.
Although this means of corrosion is almost exclusively bacterial, it is occasionally applicable
to mould.
Bacterial corrosion may be one of the following types:
a) Corrosion due to acid formation:
i) One of the main forms of corrosion is attributable to the oxidation of inorganic sulfur
compounds to sulfuric acid by the members of the thiobacillus species. These are fairly
common in soil and water. In the process, they generate and can survive in solutions
of up to 12 % sulfuric acid. Iron and concrete pipes, buildings and vulcanised rubber
are among objects thus corroded.
ii) Another form of corrosion is caused by oxidation of iron pyrites to sulfuric acid by the
ferrobacillus species. These are often found in association with thiobacillus.
Ferrobacillus is responsible for the problems arising from acid water in gold and
bituminous coal mines corroding pumping machinery.
iii) Moulds of cellulose bacteria, which ferment cellulosic material to organic acids, cause
corrosion known as 'phenol corrosion'. This can lead to the etching and pitting of
stored or buried lead and other cables and also of the paper or other cellulosic
materials within.
b) Corrosion at neutral pH values:
i) Corrosion is often brought about by cathodic depolarization, attributable to bacteria
containing an active hydrogenase. This type of corrosion is complex and its exact
mechanism is still a matter of debate, but there is no doubt that the sulfate reducing
bacteria are the cause. They are anaerobic, are commonly found in waterlogged soil or
water and some of them survive and flourish at temperatures around 40 °C. They have
been responsible for the corrosion of iron and steel in waterlogged soils, electrical
transformers, hot water systems, etc.
ii) Corrosion often occurs following differential aeration caused by the different seasonal
water levels found in flood water pipes, and is attributable to deposits formed by iron
bacteria and other micro-organisms. These form insoluble or partly-soluble products
that can adhere as films or tubercles to metal surfaces such as the inside of metal
water supply pipes.
iii) Another form of corrosion is due to bacterial metabolism. Certain bacteria produce
chemicals simply as the waste product of their digestive systems. These may be
corrosive and yet are not acids of the type discussed previously. Corrosion, of this
form, can occur remotely from the site of bacterial growth; for example the copper
heaters and valves in a high-pressure steam unit showed sulfide corrosion, although
the ambient temperature was between 120 °C and 175 °C. The corrosive material was
hydroge
...