... Part of the controversy over whether some oils are safe to use (for example Pennyroyal oil) is caused by the failure of some commentators to differentiate properly between hazard and risk. Forgive me for repeating some of the points I made in the lecture on Safety of essential oils at the AOC conference in London in July 1999, but the following points of Health & Safety policy are important to comprehend before we look at the individual properties of the oils themselves:

Hazard
something with the potential to cause harm.
(Essential oils are potentially hazardous materials)

Risk
a compound of the probability and severity of the resulting damage or harm.
(If handled in an appropriate manner, the risk involved in safe use of essential oils can be extremely small)

Banned IFRA list

Cade oil crude Juniperus oxycedrus (carcinogen)
Calamus oil Acorus calamus (some forms contain high levels of the carcinogen b-asarone)
Chenopodium ambroisoides oil (acute oral toxicity)
Costus root oil, absolute & concrete Saussurea costus (sensitiser)
Elecampane oil Inula helenium (sensitiser)
Fig leaf absolute Ficus carica (sensitiser)
(Horseradish oil) Armoracia rusticana (toxic, irritant) IFRA has banned allyl isothiocynate therefore effectively prohibiting use of Horseradish.
Mustard oil Brassica nigra (toxic; irritant)
Peru balsam Myroxylon pereirae (sensitiser) distilled oil is allowable
Sade tree oil ? further details not to hand
Savin oil Juniperus sabina. (J. phoenicea oil allowable).
Verbena oil Lippia citriodora (sensitiser)
Tea absolute Thea sinensis (sensitiser)
Sassafras albidum (carcinogen) - effectively banned by imposing a limit on the carcinogen safrole limit of 0.01% in perfumes.
Styrax gum Liquidamber orientalis and other L. spp. Only Liquidambar styraciflua var. macrophylla products produced by methods which give products not showing a potential for sensitization should be used.(e.g. steam distilln, vacuum distilln, extraction with ethanol or hexane, neutralisation with aqueous alkali followed by solvent extraction.
Wormwood oil Artemisia absinthium (neurotoxic)

IFRA Restricted list
Angelica root oil Angelica archangelica (phototoxic) 0.78%
Bergamot oil expressed Citrus aurantium ssp. bergamia (phototoxic) 0.4%
Bitter Orange oil Citrus aurantium (phototoxic) 1.4%
Cassia oil Cinnamomum cassia (sensitiser) 0.2%
Cinnamon bark Cinnamomum zeylanicum (sensitiser) 0.2%
Cumin oil Cuminum cyminum (phototoxic) 0.4%
Grapefruit expressed Citrus paradisii (phototoxic) 4%
Lemon oil cold pressed Citrus limonum (phototoxic) 2%
Lime oil expressed Citrus aurantifolia (phototoxic) 0.7%
Tagete oil & absolute Tagetes minuta (phototoxic) 0.05%
Oakmoss absolute & Resinoid Evernia prunastri (sensitiser) 0.1%
Pinaceae oils to have a peroxide value of less than 10 millimoles peroxide per litre (sensitiser)
Rue oil Ruta graveolens limit (phototoxic) 0.78%
Verbena absolute Lippia citriodora (sensitiser) 0.2%
Treemoss absolute Evernia furfuracea (sensitizer) 0.6%

First Additional list for discussion: Should we also recommend that the following oils also not be used?
Laurel leaf oil Laurus nobilis: many fragrance houses have internally imposed lists restricting or banning this product.
Thuja occidentalis oil (neurotoxic)
Tansy Tanacetum vulgare (neurotoxic)
Pennyroyal Mentha pulegium/Hedeoma pulegioides and other high Pulegone oils (e.g. Micromeria spp)
Parsley seed oil Petroselinum crispum (hepatoxic)
Parsley leaf Petroselinum crispum (hepatoxic)
Boldo oil Pneumus boldus (acute oral toxicity)
Massoia oil Cryptocaria massoia Powerful irritant.
Melaleuca bracteata oil. Methyl eugenol is a suspected carcinogen. We should not allow oils like M. bracteata (95% methyl eugenol) in aromatherapy?

from http://atlanticinstitute.com/nahasafetyrept.html

We are recommending to Conference 2000 that NAHA adopts the IFRA Voluntary Code of Practice with respect to the use of natural fragrance ingredients i.e. as it apples to the following essential oils, absolutes, resinoids and gums etc., where these items are used in massage oils. A brief listing of these is included below, but for up-to-date informinformation please visit the IFRA website. We further recommend that items in the "banned IFRA" list are not used as space odourants e.g. not to be used in nebulisers, burners etc.

We realise in making this recommendation to conference that we are going to cause some controversy, and we are not afraid of this, in fact we would far rather a positive discussion on all the issues was initiated, than for NAHA to do nothing whatsoever in giving a lead on safety issues.