ORGANISATION MONDIALE DE LA SANTE

WORLD HEALTH ORGANIZATION

INTER-REGIONAL SYMPOSIUM ON CRITERIA FOR AIR QUALITY AND ME'lliODS OF MEASUREMENT Geneva,

6-12

August

1963

ORGANISATION MONDIALE DE LA SANTE

tVrJ.OjAP/15 26 July 1963 ENGLISH ONLY

AIR QUALITY CRITERIA ASSOCIATED WITH VISIBILITY REDUCTION, SOILAGE AND DAJV'lAGE TO VEGETATION

by

John T. Middleton

Director, Air Pollution Research Center and Prcfessor of Plant Pathology, University of California, Riverside

Air is a. natural resource, vi tal to man, essential for plants and animals, and necessary for many of man's community, domestic and industrial activities. The

chemical composition of air directly affects man and the community in which he lives, the foods which nourish him, the fores-t~s ~·'hlch supply him with building material and useful products, the lands which are an integral part of his econorrcr, and the recrea- tional areas which enrich his well-being. Alterations in air quality are principally due to pollution from man's activities. Amor~ the first effects of pollution arc a reduction in visibility, soilage of property; and damage to vegetation. Tnese several effects are the harbingers of air pollution problems which may affect the health and well-being of man. For this reason, it is important to describe air qualj_ty criteria which are related to specific effects in order that ar.. evaluation of air pollution may be given, and se that appropriate control procedures may be undertaken which are in keeping with the need to maintain air of satisfactory quality.

VISIBILITY REDUCTION

Visibility, or the greatest distance one may see in a given direction, is ad- versely affected by tl1e number and kind of particles suspended in the air mass. Visi- bility is, therefore, a useful indicator of the particulate pollution loading of the atmosphere. Reduction in visibility is brought about by light absorption by large particles ranging ir. size from l to lOO microns and by forward light scattering by small-sized particles in the range from 0.3 to 0.9 microns. Particles from 0.1 to 0.3.

microns may not be ve~J important in causing significant visibility reduction, but

AP/15 page 2

they do become important in that such small partic~es are capable of absorbing organic materials and increasing their size) thereby becoming effective visibi- lity reducing agents.

The chemical composition of the particle is also an important factor in visibility reduction. An inorganic aerosol containing manganese hastens the oxidation of sulfur dioxide to §Ulfur trioxide in a moist atmosphere and effects a significant reduction of visibility. Likewise, sodium chloride nuclei bring about visibility reduction by the formation of fog droplets where relative humi- dities are 70 per cent or greater. Some of the reaction products in photochemi- cal air pollution are visibility-reducing agents; while others provide nuclei which act as centers for chemical condensation with a consequent reduction in visibility.

Criteria which describe air quality as a function of visibility reduction are difficult to formulate because of the independent er dependent factors of the physical state of the particle, its size, chemical composition, and atmospheric environmental factors of temperature and relative humidity affecting aerosol for- mation and distribution. The amount of staining of a filter paper or the darken- ing of a white surface has been used as a criterion of visibility reduction. Al- though this is a measure of particulate matter, it is not necessarily related to visibility reduction associated with forward light scattering. COH is sometimes used to describe the particulate loading of the atmosphere. COH values have been related to linear feet and equated to haze or visibility reducing values. An example of this is given below:

COHs per 1000 Visibility

linear feet rating

0 - 0.9 light

1.0 - 1.9 moderate

2.0 - 2.9 heavy

3.0 -- 3.9 very heavy

4.0 plus extremely heavy

Several studies have been made which report the particulate loading of a number of rural and urban areas. One of these, conducted in the United States, has shown that there is a considerable range in the amount of suspended particu- late matter from as little as 10 micrograms per cubic meter to as much as 1000

J.lg·

About 50 per cent of the samples from rural areas showed approximately 35

IJ.g

^per

cubic meter while 50 per cent of the samples from urban areas showed about 150 Jig per cubic meter.

. '• ~11,

AP/15

page 3

While it can be shovm that there may be a specific relationship between visibility a.n.d particulatc loading in certain specific sampling locations, there

is no evidence; that such an association has general applicability. For this rea-- son, an air quality criterion for visibility would best be related to a visible standard of a specific distance rather than to n gross measurement of a quantity of particles. The distance selected should be determined on the specific require- ments of the particular area and its conm1Uni ty needs. It is also important that the criterion for visibility be separated from th•:: natural phenomenon of haze in the area considered. Although a visibility criterion may be set on the basis of aesthetics, it may be more reasonable to establish it on visibility requirements for aircraft operation. A criterion for ,~desirable visibility in tl1c State of California is dcscribc::d as less than 3 miles with the atmospheric relative humi- dity at 70 per cent or less.

SOILAGE

The establishment of air quality criteria for soilage is fraught with many difficulties because soilage is typically the result of a complex of reactions rather than the result of a single specific contaminant effect. While soilage anddarr.£tge to property can sometimes be specifically associated with a recognized contaminant, such as ozone causing cracking of stretched rubber, soilage usually is brought about by interaction of the cnvironment and a variety of contaminants that cause abrasion, chemical deterioration, and corrosion of buildings, equipment and goods.

Carbon dioxide, sulfur oxides, and hydrogen sulfide are the most common causcs of a very large percentage of the soilagc and deterioration of ma.terials.

The damage resulting frotn these three contaminants is dependent upon atmospheric moisture and tempera tur<:·. Because the damage produced is the result of the inter- action of the toxicant, the:: status of the:: substrat.:c and the c:nvironment) an air quality criterion for soilagc cannot genc::rally be accurately determined. An exam- ple of this complexity is illustrated by the feasibility of establishing an air quality criterion for ozone cracking of rubber products. Ozone specifically attacks some types of rubber when it is stretched undcr stress. Rubber suscepti- bility ~o ozone damage varies with the dc::grce of bond saturation and location of chlorine atoms. The amount and depth of cracking of a particular rubber is direct- ly proportlonal to the exposure timE: and toxicant concentration. The extent of damage is also a function of the amount of imposed stress and the:: temperature. An

AP/15 page 4

air quality criterion for the soilage of rubber could be established only if the modifying factors arc adequately described. A criterion describing threshold damage to susceptible rubber under certain conditions might be 0.1 ppm ozone for 1 hour, and under others as much as 1.0 ppm for 1 hour.

Ozone may also be used as an example of a toxicant that has an adverse effect upon fabric strength and dye fastness. An air quality criterion for ozone and damage to fabrics and dyes may be established as more is learned of the environ- mental factors modifying ozone effects.

It is possible to relate the soiling of cloth and other surfaces to air-borne particulates. This may be done by relating the COR values found in the air enve- loping stained or discolored goods, buildings 2 and wearing apparel.. The air quality criterion would perhaps best be described as the micrograms of particulate matter per cubic meter of air which discolor a given surface for a specified time period.

Considerable attention has been given to the corrosion of aluminium, iron, painted surfaces, and silver electrical contacts. The damaging effects of sulfur dioxide on ironhavebeen extensively investigated and the amount of deterioration related to the length of exposure₂ the hours of surface wetting, air temperature, and the concentration of sulfur dioxide. Such data can be used to develop an air quality criterion but only when conditions ambient the affected metal are stipulat- ed. Information is also available which shmvs the relationship of hydrogen sulfide to the soiling of lead-white painted surfaces. Darkening is a function of concen- tration, exposure tirne₂ and duration of surface wetting. Since discoloration oc- curs at levels as low as those causing odor nuisance, perhaps the criterion for soilage may be related to the odor criterion obnoxious to man and described as 0.1 ppm for only a few minutes.

DAMAGE TO VEGETATION

Air quality criteria may be established for damage to vegetation because most plants respond differently to different specific contaminants. The effects pro- duced and the amount of injury are functions of dosago2 thus permitting the develop- ment of a criterion which expresses time and concentration. With few exceptions, the dosages required to damago vegutation occur at or below those levels which pro- duce adverse effects on humans. A notable exception is carbon monoxide which has no effects on vegetation2 even at 1000 ppm for 1 hour.

Particulates

A.P/15

page 5

Liquid and solid particulatcs often have adverse effects upon vegetation through the production of necrotic lesions due to the direct toxicity of the material or by reduced plant growth through indirect effects of relatively inert particles, such as cement and gypsum dusts. Air quality criteria relating the effects of particulatcs to vegetation damage have not been developed, primarily because of the difficulty in describing the 2nvirrmmcntal factors determining the particle size; factors regulating leaf wettability and surface contact times re- quired to produce measurable effects.

Sulfur dioxide

Air quality criteria have been established for sulfur dioxide effects upon vegetation because of extensive knowledge n:sul ting from the large amount of re- search given this toxicant since it was first recognized almost lOO years ago to be an air-borne contaminant of importance to agriculture. The acute effects of sulfur dioxide are recognized by the production of light tan-to-brown necrosis along the margins of leaves and extending inward between the veins. The relative susceptibility of plants to sulfur dioxidC: can be readily demonstrated by fumiga- tion and the extent of injury associated with dosage. These data may then be used to develop threshold or complete damage dosage levels. The amount of damage is a function of varietal susceptibility, thus enabling the development of damage constants for affected species. Through fumigation studies, it has been deter- mined that the most susceptible species arc alfalfa; cotton and barley. Assign- ing alfalfa a constant cf 1₃ more resistant varieties arc assigned larger constants3

such that corn receiV8S a constant of

4.

The threshold damage level for alfalfa is T(C-.2)

=.9

3 where T is time in hours and C is concentration in ppm. The formula for complete damage is T(C-2.6)

=

).2. The air quality criterion for threshold damge for alfalfa then is about l ppm sulfur dioxide for 1 hour, while the complete damage criterion is abou+ 6 ppm fc;r l hour; those values for corn are 4 times greater. The values for complete plant damage at l hour arc comparable to levels known to be harmful to humans.

Fluorides; especially gaseous hydrogen fluoride and silicon tetra-fluoride, are important toxicants causing damage to vegetation.

feed levels exceed about 50 ppm on a dry vmight basis.

Livestock are damaged when Whereas damage to vcgeta- tion from fluorides depends 11pon tho absorption of the toxicant from gas or soluble particulates, the injurious effects upon livestock result from both the contained fluoride and any surface residue on the forage. Fluoride damage to

/AP/15 page 6

vegetation characteristically causes light brown-to-red-brown marginal and apical leaf necrosis. Hydrogen fluoride is typically absorbed by the entire leaf surface and then migrates to the margin and apex of the leaf, accounting for fluoride con- centrations ranging from 2 to 100 times that in healthy leaf tissue. Fluoride levels in leaves far exceed those in the atmosphere because the leaf acts as an accumulating system permitting the concentration of fluoride in tissue. The

damaging effects to vegetation, therefore, are due to the accumulation of fluoride and not to the direct response of the ambient dose.

There is a great range in susceptibility of plants to fluoride toxicity. Leaf injury is noticed on gladiolus and apricot when the fluoride levels for the entire leaf range from 30 to 40 ppm. No injury is seen when 300 to 500 ppm are present in apple, while cotton leaves remain healthy despite the accumulation of 5000 ppm.

The environment in which the plant is exposed to the fluoride affects the amount of damage. Plants grown under water stress, nitrogen deficiency, and at high air temperatures show adverse damage effects much more promptly than do plants grown under more ideal situations. There is also considerable variation to fluoride

toxicity within varieties. For example, Mission grape leaves show visible leaf necrosis when they contain about 150 ppm fluoride, while leaves of Concord grape show no damage when they contain about 750 ppm fluoride. Another variable which must be considered in developing criteria for atmospheric fluoride contamination is that different plants accumulate fluoride at different rates, even when exposed to the same ambient level of hydrogen fluoride. For example, when apricot and carrot were exposed to 1.5 parts per billion for 170 hours, carrot leaves accumu- lated 5 times more fluoride than did apricot.

Other effects are noticed on vegetation from the accumulation of fluoride and include alterations in growth habit ~~ith the stimulation of adventitious buds giving plants a conspicuously branched habit. Fluoride accumulation in plants without necrosis has also been noted to cause excessive flower and leaf drop, thereby possibly affecting crop production and plant growth. In addition to these alterations, there are chronic effects which are primarily manifested by chlorosis in leaves. These several effects have been produced experimentally in lemons by controlled fumigation with hydrogen fluoride at 1 part per billion for several months. Many of these adverse growth effects occur at the same atmospheric fluoride levels as do visible injurious effects but typically require longer ex- posure periods. Such an air quality criterion will, therefore, require

iAP/15 page

7

consideration of a very low concentration value, perhaps 0.1 ppb, and a long time period, perhaps several weeks.

The significant and adverse effects of fluoride accumulation in vegetation in causing damage to crops and fluorosis in livestock certainly suggest that an air quality criterion be established for hydrogen fluoride and for other fluo- rides as they become identified in the atmosphere. Variables in plant growth, the external environment apart from contaminant level, the extremes in suscepti- bility of a variety of crops~ the differences in fluoride accumulation rates in vegetation, as well as alterations in growth apart from necrosis, preclude the development of an air quality criterion at this time. These recognized factors affecting the accumulation of fluoride in vegetation, however, suggest areas re- quiring research so that information will become available permitting description of fluoride air quality criteria.

Either as a contaminant from the loss of manufactured gas, from chemical manufacture, or from the combustion of motor vehicles, ethylene is a contaminant which has long been known to affect fruit maturity and colour, leaf drop, leaf

form, and flower damage. Ethylene is intentionally used to mature some fruits, such as citrus, where fruits arc exposed from 1 to 2 days to 10 to 15 ppm.

Leaves of rooted roses are typically removed by exposing plants to about 5 ppm for 8 hours. Leaf malformation in glasshouse-grown tomatoes readily occurs when plants are inadvertently exposed to 0.1 ppm for about 8 hours. In addition to causing leaf cpinasty in tomato, ethylene at 0.1 ppm for 8 hours or onger will cause significant flower drop in tomato and pepper.

One of the more important deleterious effects of ethylene is the damage it causes to flowers of carnation, orchid and snapdragon. The flo1vcrs and buds of snapdragon arc readily dropped when ths plants arc exposed to 0.5 ppm for l hour.

The flower buds of carnation fail to open properly and lose turgor when exposed to 0.1 ppm for

6

hours. Necrosis of orchid sepals is induced by exposure of flowers to as little as 0.01 ppm for 24 hours, while similar effects arc produced with 0.5 ppm for

6

hours or 0.3 ppm f•r 1 hour. An air quality criterion for ethylene as related to flower production may be established as 0.5 ppm for 1 hour.

Photochemical air pollution

A variety of kinds of vegetation damage results from the several toxicants present in photochemically polluted air. With the exception of ethylene, none of the hydrocarbons participating in photolysis with nitrogen dioxide cause plant

AP/15 page 8

injury at concentrations below 100 ppm. Nitrogen dioxide) the photoacceptor, initiating the photochemical reaction of hydrocarbons in the presence of sunlight, produces an acute symptom similar to those caused by sulfur dioxide when it is present 4 hours or longer at about 5 ppm. Growth suppression and chronic effects resultto sensitive plants when they are exposed to nitrogen dioxide at 0.2 ppm for 2 weeks under certain growth conditions. The more important vegetation damaging agents resulting from the photolysis of N0

2 and hydrocarbons are ozone and oxidants.

Peroxyacyl nitrates

The oxidants in photochemical air pollution have been described as the reac- tion products of the dark reaction of hydrocarbon with ozone, and the light reac- tion of nitrogen dioxide with hydrocarbons. The most important plant damaging oxidants are the peroxyacyl nitrates. Three have b8en identified: they are peroxyacetyl nitrate, peroxpropionyl nitrate, and peroxybutryl nitrate. These toxicants produce glazing and bronzing of the lower leaf surface of a large num- ber of herbaceous and woody plants. In addition the toxicants induce growth sup- pression and leaf drop. An 80 per cent level of injury can be produced on sensi- tive bean leaves at the following dosages: l ppm for 15 minutes, 0.4 ppm for 30 minutes, or 0.2 for l hour, 0.1 for 2 hours, 0.05 for 4 hours, or 0.025 ppm for 8 hours.

As with most other plant-damaging toxicants, there is a wide variability in the susceptibility of plants to the peroxyacyl nitrates. Whereas tomato leaves show damage after 2 hours' exposure to 0.025 ppm, it requires

7

hours exposure to 5.0 ppm to damage citrus with peroxyacyl nitrate (PAN). Not only is there a difference in susceptibility of plant species_, but there is a difference of sus- ceptibility based on age of leaf. Bean leaves are most susceptible when they are 5 to 6 days old, gradually becoming resistant with age. Further, the environment at time of exposure is important. A plant susceptible to PAN must have been .in

the light before, during and afterwards, otherwise injury will not occur. Here as with fluorides, the growing conditions of the plant are important modifiers of

:.>

its susceptibility. Plants grown under optimum conditions, unlike fluoride, are most susceptible to oxidant &•mage, whereas those grown under adverse conditions are most resistant.

The severity of plant damage also depends upon the kind of peroxyacyl nitrate.

A limited number of experiments have shown that whereas 0.025 ppm of peroxyacyl

•AP/15 page 9

nitrate will cause injury after 2 hours' exposure, the same amount of da~~ge will occur at the same concentration of peroxypropionyl nitrate in

V2

hour and from peroxybutryl nitrate after but

V4

hour.

In addition to the visible injuries produced, PAN also produces effects upon growth, photosynthesis, and respiration. PAN at

1.3

^{ppm for}

3

hours inactivates indole e.cetic acid by altering the indole ring. The growth suppression effects of PAN are prevented by the addition of ascorbic acid and reduced glutathione in test systems wherein PAN suppresses growth of tre Avena colcoptilc . The addition of ascorbic acid and glutathione to inactivated indole acetic acid did not reinstate its activity. Although the effect of PAN on indole acetic acid is real, this effect is not the cause of the growth suppression. vlhereas l.l ppm of PAN for

4

hours will cause a 32 per cent reduction in growth of Avena coleoptilc, 2.4 ppm for 4 hours causes an 84 per cent reduction. At levels above l ppm PAN has been shown to give a 90 per cent inhibition of incorporation of carbon 14 into cellulose and acid soluble wall fractions. It also causes a 40 per cent decrease in carbon dioxide evolution and a significant decrease in the amount of water-soluble pectin and some polysaccharides. Since PAN appears to have a minimal effect on respiration but a significant effect on growth, it is suggested that the material has specific

effects on enzymes involved in growth and on the metabolism of polysaccharides. The effect of PAN on other enzyme systems is now under study, particularly those which participate in lipid synthesis. As these biochemical studios progress, air quality criteria may be established which relate not only to the visible effects of damage to vegetation but perhaps more importantly to the development of air quality criteria that arc directly related to significant biochemical function. An air quality

criterion for PAN may well be established at 0.2 ppm for 1 hour as causing signifi- cant effects on plants and possibly causing irritation to human eyes.

Ozone

Ozone, unlike oxidant, produces visible effects by damaging the upper leaf surface of most herbaceous and some woody plant species. These symptoms are con- spicuously different from that induced by oxidant in that the upper loaf surface is typically spotted, chlorotic, and bleached. If the damage is extensive, then the tissues become brown and most of the tissue is affected and may inv.olve the entire leaf. Again there are large differences among plant species with regard to their susceptibility to ozone as well as differences in their response, depending upon the environment in which the plant is grown and exposed to the toxicant. Suscepti- ble sensitive plants~ such as bean and tomatoJ as well as the needles of pine, are

·AP/15 page 10

easily damaged by low concentrations for short periods of time. The damage is a function of concentration and time of exposure but the dosage effect is not always linear. This is particularly well shown by tte fact that almost complete damage of bean leaves was possible at 1.1 ppm for 15 minutes while the same amount of

in.jury occurred at 0.6 ppm for 30 minutes, 0.4 for 1 hour, 0.3 for 2 hours, 0.3 for 4 hours, and 0.25 for 8 hours. Significant damage to susceptible bean leaves can be caused by 0.5 ppm for only 10 minutes, providing the bean leaves are of suitable age and that they have received adequate light before and during fumiga- tion. Whereas bean leaves were most susceptible to PAN when they wer~ 5 to 6 days old, they must be

9

to 11 days old to be damaged by ozone. Unlike PAN, plants need not be exposed to light after fumigation in order to induce ozone damage. However, susceptible beans must have adequate light before fUmigation and possibly during that time in order to assure symptom development. Apparently, the basis for injury is the need for a sugar content of 4.0 t() 1.0 micrograms per gram of fresh leaf tissue.

Threshold damage to pine occurs at 0.1 ppm after 2 hours while 0.2 ppm for 2 hours is usually required for threshold damage to alfalfa, broccoli, spinach and tobacco. The importance of environmental conditions is shown by the fact that annual blue grass under certain circumstances may be readily injured by 0.15 ppm for 1 hour, whereas under other conditions, 0.64 for 2 hours are required.

In addition to the visible toxic effects produced by ozone at concentrations mentioned above, leaf drop, chlorosis, and adverse effects result when plants are exposed to 0.1 ppm or less for 1 or more weeks. These effects are believed to be brought about by adverse effects upon plant metabolism. This is demonstrated by the fact that when mitochondria extracted from spinach leaves and cow's liver are exposed to 1 ppm for a short time, their activity is greatly altered with a signi- ficant increase in respiration potential. The effect of~one upon mitochondrial activity can be overcome through the addition of reduced gJutathionc and ascorbic acid. However, these materials either must be present \'li th the mitochondrial pre- paration at the time of fumigation or introduced prior to the point of irreversi- bility of the narcotic action. The application of ascorbic acid, as well as a number of other reducing agents such as the di thiocarbatrk.'l. tes, to leaf tissue pre- vents the leaf from damage by ambient concentrations of ozone, perhaps suggesting that sulfhydryl enzyme systems form the basis of a mechanism injury induced by ozone.

AP/15 page 11

Citrus issusc2ptible to ozone, both through excessive leaf drop at low con- centrations for long periodsof time and higher concentrations for shorter periods of time. When citrus is exposed to 0.2 ppm for 8 hours for 5 days" the carbohy- drate metabolism is significantly .:-tlte:rcd. After 10 days, the respiration rate is doubled. Fifteen days of such exposure increased the utilization of carbon in the sixth position and decreases the utilization of first carbon in sugar forma- tion. After 40 days of such exposure there is tissue damage as well as a 70 per cent reduction in carbohydrate9 a 46 per cent reduction in starch, and an increase in the amount of reducing sugars, but with no change in sucrose levels.

Ozone has significant effects on vegetation and is responsible for visible damage in the form of leaf necroses, the loss of leaves, and the reduction in plant size. In addition to visible effects, there are biochemical ones which primarily affect respiration by increasing it and thereby significantly altering the carbohydrate status of tissue. Damage from ozone is related to the concentra- tion of the toxicant and duration of exposure permitting the establishment of air quality criteria for groups of plants of similar susceptibility. A no~inal

criterion may be 0.2 ppm for l hour" a concentration having noticeable and ob- noxious odor effects on humans.

SUMMARY

Deterioration of air quality typically results from the pollution of the atmosphere through man's industrial and domestic activities. The release of con- taminants into the air space, their reaction, and their transport result in reduc- tion in visibility, soilage of goods, and damage to vegetation. Air quality cri- teria relating these effects to concentration and exposure time may be readily established for specific contaminants which have direct effects.

Particulate loading of the atmosphere is primarily responsible for reduction in visibility. The amount of visibility reduction, in turn, is due to the nature of the particle, such as its size, color, ~d chemical composition. Whereas visi- bility reduction in specific cases may be directly related to the amount of parti- culate matter per linear foot, there is no general application of this to wide- spread communities. The criterion for visibility is perhaps best given in terms of visible distance at a specific relative humidity.

Air quality criteria for soilage may be developed where specific contaminants have direct effects upon particular ~~terials. Since in general the soilage of

AP/15

page 12

goods is a function of multiple actions, of not only the air contaminants but the goods and the environment in which they are situated, air quality criteria for soilage are difficult of description and perhaps of lessened general utility.

Air quality criteria for visible effects of vegetation damage can be deve- loped for those toxicants which have direct effects, such as sulfur dioxide, ethy- lene, nitrogen dioxide, peroxyacyl nitrates, and ozone. The criteria for these toxicants may be adequately described in terms of concentration and exposure time for particular effects. Air quality criteria for fluorides cannot now be deter- mined because of the multiplicity of factors which control the rate of accumula- tion and the subsequent effect of fluoride upon plant tissues. Levels of fluoride responsible for damage to livestock can be established and a criterion based on this is feasible.

With few exceptions, visibility reduction, soilago of goods, and damage to vegetation all occur at concentrations and exposure times below those recognized as having significant effects upon man. These criteria, therefore, become impor- tant and useful in describing the quality of air and recognizing the destruction of the air resource in describing the air pollution problem in advance of signi- ficant effects upon humans.

CRITERES DE LA QUALJTE DE L¹AIR QUI SONT ASSOCIES

A LA DIMINUTION DE LA VISIBILITE, A LA CONI'AMINATION DES DENREES ET AUX DOMMAGES CAUSES A LA VEGETATION

par

John T. Middleton

Director, Air Pollution Research Center, et Professor of Plant Pathology, University of California, Riverside

RESUME

fiP/15 pAge l3

La deterioration de la qualite de l ¹air resulte essentiellement de la pollution de l ¹atmosphere par certaines activites humaines, industrielles et do- mestiques. La liberation de contaminants dans l ¹air ambiant, leurs reactions et leur transport provoquent une diminution de la visibilite, une. contamination des denrees et des dommages

a

la vegetation. Il est aSSQZ facile d¹etablir des cri- teres de la qualite de 1¹air mettant ces effets en relation avec la concentration et avec le temps d¹exposition lorsque la pollution est due

a

des contaminants spe- cifiques qui exercent une action directe.

La charge particulaire de l ¹atmosphere est la principale cause de la diminution de visibilite. L¹importance quantitative de cette diminution,

a

^son

tour, est fonction de la nature des particules : grosseur, couleur, composition chimique, etc. Tandis que la diminution de la visibilite peut, dans certains cas,

@tre mise en correlation directe avec la quantite de maticre particulaire presente par pied lineaire, un tel critere ne peut en general s'appliquer

a

des communautes dispersees. La meilleure fagon d¹evaluer la visibilite est peut-@tre de l'exprimer par la distance

a

laquelle des objets peuvent @tre vus

a

un degre determine d¹humi- dite relative.

AP/15 Page 14

Des criteres de la qualite de 1¹air associes

a

la contamination des denrees peuvent @tre elabores lorsqu¹on se trouve en presence de contaminants specifiques qui exercent une action directe sur les matieres particulaires. Comme, en regle generale, la contamination des denrees resulte de facteurs multiples - non.seulement les conta- minants de l'atmosphere, mais aussi les denrees elles-m@mes et le milieu dans lequel elles sont situees -, les criteres associes

a

cette contamination sont difficiles

a

circonscrire et presentent peut-@tre une utilite moins generale.

Des criteres de la qualite de 1¹air associes aQX dommages visibles causes

a

la vegetation peuvent aussi @tre fixes pour les agents toxiques qui ont une action di- recte, par exemple le gaz sulfureux, l ¹ethylene, lP dioxyde d¹azote, les peroxyacyl- nitrates et l ¹ozone. Ces criteres s'expriment de fagon adequate en concentration ou en temps d¹exposition necessaires

a

la production d¹un effet determine. Il n¹est pas encore possible, au stade actuel, d¹etablir de tels criteres pour les fluorures en raison de la multiplicite des facteurs qui interviennent aussi bien dans le taux d'accumulation que dans 1¹action ulterieure du fluorure sur les tissus vegetaux, Par contre, les quan- tites de fluorure capables de nuire au cheptel peuvent @tre determinees et un critere etabli sur cette base.

A quelques exceptions pres, la diminution de la visibilite, la contamination des denrees et les dommages

a

la vegetation surviennent tous

a

des concentrations et

a

des temps d¹exposition inferieurs

a

ceux dont on sait qu¹ils exercent des effets notables sur 1¹homme. Aussi les criteres associes

a

ces phenomenes presentent-ils un inter@t cer- tain pour l'evaluation de la qualite de 1¹air comme pour le depistage de la deterioration du milieu atmospherique ils permettent en effet de jauger les problemes de pollution atmospherique avant que ceux-ci n'exercent une action sensible sur l ¹homme.