Michael Russell

Wollongbar Agricultural Institute, Wollongbar, NSW, Australia

INTRODUCTION

For some essential oils, as little as one teaspoonful can be fatal. Wormseed, sassafras, parsley, eucalyptus and camphor have all caused child fatalities in low doses (Tisserand and Balacs 1995). Consequently, guarding against accidentally overdosing with certain essential oils is important, especially with children. Storing essential oils that are known to be harmful out of children's reach and in child-proof bottles is a sensible way to minimise accidental consumption. Nevertheless, it is of vital importance to understand the toxic properties of each specific oil to be certain of the risks involved.

Toxicity can be manifested by many symptoms, beginning with a rash and redness of the skin, extending to organ damage, especially the liver and, in the extreme, resulting in death. Toxicity is generally dose dependant, with oral intake more significant than dermal absorption. Also, acute toxicity results from higher doses than those responsible for longer term chronic problems.

In this chapter, toxicology will be discussed in relation to acute oral and dermal toxicity, to dermal irritancy and to dermal allergy responses. The amount of an essential oil needed to bring about a response in each of these broad categories varies greatly. This variation is due to individual human tolerance (oral or dermal), dose frequency and the oil's intrinsic toxicity.

TEA TREE OIL—MELALEUCA ALTERNIFOLIA Acute Oral Toxicity

Acute oral toxicity of a substance is measured by the LD50 test which measures the effect of differing amounts of a test substance given orally to a group of animals (usually rats or mice). The group is of uniform age, size and genetic origin. Hence LD50 measures the dosage which is lethal to 50% of the test animals.

The LD50 test is probably the prime result for any substance as it determines whether or not the substance can be ingested safely or not. LD50 test results are usually expressed in the form of grams (or ml) per kilogram of body weight, thus the heavier the person, the higher a dose needs to be, before it becomes lethal. It is for this reason that children are particularly susceptible to lethal poisoning. To help appreciate the significance of the LD50 test, Table 1 converts LD50g/kg results for common essential oils to a lethal dose for a 70kg adult and a 10kg child.

Tea tree oil has been reported to have an LD50 of 1.9g/kg (Ford 1988; Tisserand and Balacs 1995; Table 1), 1.9-2.6ml/kg (or 1.7-2.3g/kg for an oil of density 0.9) (Bolt 1989d; Altman 1991) and 3.0ml/kg (or 2.7g/kg) (Austteam 1995). Tisserand and Balacs (1995), although classifying tea tree oil with oils on the borderline of safe and unsafe, place k at the safe end of that group and state that "we have no reason to suspect any problems with tea tree".

Table 1 Some well known essential oils and components with their LD50 values (g/kg) in Rodents, from Tisserand and Balacs (1995) with corresponding calculated doses (gm) for a 70kg adult and a 10kg young child

OIL

(70 kg aduit)

IAo doss (g) (10 kg child)

GROUP 1

Hydrocyanic acid

0.0001

0.007

0.002

Boldo leaf oil

0.13

9.1

1.3

Worm seed oil

0.25

17.5

2.5

Mustard oil

0,34

23.8

3.4

ldh

Pennyroyal oil (Eur.)

0.4

28

4

Camphor (chemical)

0.5-15

35-1050

5-150

Tansy oii

0.73

51.1

7,3

Thuja oil

0.83

58.1

8.3

Almond (bitter) oil

0.96

67.2

9.6

GROUP 2

Oil of Wintergreen

1.20

84

12

Cornmtnt

1.25

87.5

12.5

Hyssop oil

1.4

98

14

Almond (bitter) FFPA*

1.49

104.3

14.9

LDso

Sassafras (braz.)

1.58

110.6

15.8

1—>2

Myrrh oil

1.65

115.5

16.5

Oregano

1.85

129.5

18.5

Sassafras

1.9

133

19

Tea tree

1.9

133

19

GROUP 3

Anise

Cajeput

Coriander

ldso

Cinnamon bark

between

between

2—>5

Eucalyptus

150 and 350

20 and 50

Neroli

Spike lavender

GROUP 4

Bergamot

Chamomile

Citronelli

above

above

LDW

Geranium

350

50

5—>

Lavender

Sandalwood

Ylang-ylang

*FFPA=Free From Prussic Acid.

*FFPA=Free From Prussic Acid.

The LD50 test for tea tree oil conducted by Bolt (1989d) using OECD method 401 as per the Australian Tea Tree Oil Toxicology Data Sheet No. 5 (Tea Tree Oil Growers of Australia 1989) was performed as follows:

Tea tree oil was diluted with peanut oil and tested at 20%, 25% and 33% concentrations using groups of 5 male and 5 female Sprague Dawley SPF (specific pathogen free) and non-SPF rats. Animals received doses of 1.7-3.0ml/kg by gavage and were observed for 14 days. The LD50 was found to be 2.6ml/kg in SPF rats and 1.9ml/kg in non-SPF rats. Ingestion of pure tea tree oil is not recommended.

For the animals surviving the above treatment, the major non lethal reaction was a complete lack of muscular tone in the forelimbs, bloodied noses and weeping (Bolt 1989d).

These results suggest that tea tree oil is less toxic than commonly used oils like pennyroyal (LD 50 0.40), wintergreen (LD50 1.20), cornmint (LD50 1.25), basil (LD50 1.40), and bay leaf (W. Indian) (LD50 1.80), (Tisserand and Balacs 1995).

Poisoning

Essential oils in general are a toxic risk especially to children and old people. Cases of child poisoning due to overdose of essential oils such as camphor, cinnamon, citronella, clove, eucalyptus, pennyroyal, sassafras and oil of wintergreen have been reported (Tisserand and Balacs 1995). In America in 1973, 500 cases of camphor intoxication alone were reported. Most of these were young children given camphorated oil instead of castor oil. In one instance, a 16 month old boy died from consuming one teaspoon of 20% camphorated cottonseed oil (Smith and Margolis 1954) thus making about 1 gram of camphor the toxic dose to children. The chemical camphor is up to 60-70 times more toxic to humans (LD50 0.005-0.5g/kg) than to rodents (LD50 0.5-15g/kg) (Tisserand and Balacs 1995, Table 1).

As the oral toxicity LD50 test is carried out on rodents and not humans the results can be misleading. Take for instance oil of wintergreen (methyl salicylate). In six cases of poisoning from the oil in adult humans, three people died from ingestion of 15ml, 30ml and 80ml and three survived after 6ml, 16ml and 24ml (Stevenson 1937) giving an average oral lethal dose of 0.2g/kg to 0.3g/kg for humans. The LD50 however, is 1.2g/kg for rodents suggesting that oil of wintergreen is some three to five times more toxic in humans. According to Opdyke (1977) an oral dose of 4-8ml of methyl salicylate is considered lethal for a child.

Similarly, eucalyptus oil has shown itself to be fatally toxic to humans in amounts between 30ml and 60ml (Gurr and Scroggie 1965) thus making it 4 times more toxic to humans than to rodents. With these figures in mind, one might be tempted to say that LD50 tests are of limited value. The lesson here is that with the LD50 test, one has a comparative assessment of an essential oil's oral toxicity potential in humans.

Furthermore, with tea tree oil being in the borderline category when tested with rodents (Tisserand and Balacs 1995), it is probably prudent to assume that tea tree oil is orally toxic in reasonably large doses, so oral ingestion should be avoided. This recommendation is even more applicable to children, who should be kept well clear of any container of essential oil including tea tree oil.

This is well illustrated by the known cases of tea tree oil poisoning in humans (Carson and Riley 1995).

Del Baccaro (1995) cites a case of a 17-month-old male child who developed ataxia and drowsiness after ingestion of less than 10ml of oil of Melaleuca alternifolia. With treatment of activated charcoal and observation, the child was discharged 7 hours later.

A similar case involving a 23-month-old boy was reported by Jacobs and Hornfeldt (1994):

A 23-month old boy became confused and unable to walk thirty minutes after ingesting less than 10ml of T36-C7, a commercial product containing 100% Melaleuca oil. The child was referred to a nearby hospital. His condition improved and he was asymptomatic within 5 hours of ingestion. He was discharged to home the following day.

Adults are also susceptible with one sixty-year-old male, after ingesting half a teaspoonful of oil, reacting with severe dermatitis in addition to feeling quite unwell (Elliot 1993). This was thought to be "systemically induced eczema or a cutaneous reaction to an ingested contact allergen" (Moss 1994).

On the other hand, one adult became comatose for twelve hours and semi-conscious for a further thirty-six hours after ingesting half a cup of neat tea tree oil (Seawright 1993). Despite abdominal pain and diarrhoea which persisted for six weeks, this patient survived a dosage estimated at 0.5-1.0ml/kg body weight. Hence tea tree oil can be considered safer than many other oils, including camphor, wintergreen and eucalyptus oils.

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