Hey apaul I googled it for you and got this link-->
http://www.organicfacts.net/organic-oils/natural-essential-oils/list-of-essential-oils.html
As far as side effects go, there can be many factors involved, But if you just looking at witch one will sensitize the skin here is a link and some info i pulled off someones ebay page. -->
http://www.essentialoils.co.za/irritation.htm#Essential oils that may cause skin irritation
Never, Never, NEVER Use These Essential Oils
Unless under the supervision of a certified and registered Aromatherapist, do not even be tempted to smell these oils!
Ajowan
Trachyspermum copticum - dermal (skin) irritant, possibly irritating to mucous membranes (ex. the membranes inside your nose), and must not be used during pregnancy
Almond, Bitter
Prunus dulcis - contains prussic acid, also known as hydrocyanic acid or cyanide, is a well-known poison; also contains the moderately toxic benzaldehyde
Allspice, Leaf and Berry
Pimenta dioica - dermal (skin) irritant; and, due to the eugenol content, very irritating to mucous membranes (ex. the
membranes inside your nose)
Anise, Star
Illicium verum - although it does not seem to be toxic when used as recommended, if you use too much it can slow down your circulation and lead to cerebral (brain) disorders, so it is best to avoid it alltogether
Many retail stores sell star anise essential oil despite the safety concerns - do not assume that just because you see it on the retail shelf that it is safe!
Aniseed
Pimpinella anisum - causes dermatitis, and, like star anise, if you use too much it can slow down your circulation and
lead to cerebral (brain) disorders, so it is best to avoid it alltogether
Arnica
Arnica montana - the essential oil is highly toxic, however tinctures or homeopathic remedies are safe and very useful
Basil, Exotic
Ocimum basilicum - these safety cautions only apply to exotic basil, and not to French basil (although they have the same latin name, they are made up of dramatically different constituents); moderately toxic, dermal (skin) irritant, and may be carcinogenic (causes cancer)
Birch, Sweet
Betula lenta - although the methyl salicylate contained in sweet birch isn't exactly toxic, if you use too much it can be very harmful and so best to avoid altogether
Boldo
Peumus boldus - extremely toxic
Broom, Spanish
Spartium junceum - toxic due to the sparteine content
Buchu
Agothosma betulina - should not be used during pregnancy or be used around anyone who is pregnant
Calamus
Acorus calamus var. angustatus - an oral toxin, plus strongly suspected of being carcinogenic (causes cancer)
Camphor, Brown and Yellow
Cinnamonomum camphora - brown and yellow camphor contain safrol and are toxic and carcinogenic (causes cancer); white camphor is relatively non-toxic
Cassia
Cinnanomum cassia - dermal (skin) toxin, irritant, and sensitizer; and very irritating to mucous membranes (ex. the membranes inside your nose)
Watch out for cassia essential oil - I don't know why, but it is popularly used by handcrafted soapmakers (who are unaware of the risks), especially in abrasive (exfolliating) soaps
Cedarwood, Texas or Virginian
Juniperus ashei and Juniperus virginiana, repectively - use Atlas cedarwood instead, as these two kinds of cedarwood can cause acute local irritation
Chervil
Anthriscus cerefolium - strong irritant and toxin, highly suspected of being carcinogenic (causes cancer)
Cinnamon, Bark
Cinnamomum zeylanicum - the leaf oil is relatively non-toxic, but the bark essential oil is a dermal (skin) toxin, irritant, and sensitizer; it is also very irritating to mucous membranes (ex. the membranes inside your nose)
I have heard of beauty magazines publishing homemade lipbalm recipes calling for the spice cinnamon - cinnamon spice is made from the bark, and I have had to explain to people that this is why the lip balm made their lips burn!
Clove, Leaf and Stem
Syzygium aromaticum - dermal (skin) irritant and very irritating to mucous membranes (ex. the membranes inside your nose); clove bud essential oil is a less toxic alternative because it contains less eugenol (but still use cautiously)
Costus
Saussurea costus - severe dermal (skin) sensitizer (causes allergic reactions in people who do not have allergies)
Deertongue
Carphephorus odoratissimus - anecdotally has caused liver damage and hemorrhages, plus the lactones can cause dermal irritation
Elecampane
Inula helenium - although it is non-toxic, it is a severe dermal (skin) sensitizer (in clinical tests it caused "extremely severe allergic reactions" in 23 out of 25 volunteers)
Fennel, Bitter
Foeniculum vulgare - causes sensitization (allergy-like symptoms) and should never be used on the skin - if you must use fennel, make sure it is sweet fennel (not bitter fennel)
Horseradish
Armoracia rusticana - an oral toxin, dermal (skin) irritant, and very irritating to mucous membranes (ex. the membranes inside your nose)
Jaborandi
Pilocarpus jaborandi - an oral toxin, skin irritant, and will cause medically dangerous spontaneous abortions in pregnant women
Melilotus
Melilotus officinalis - in 1953 many countries, including the USA, banned Melilotus from use due to toxicity levels
Mugwort
Artemisia vulgaris - an oral toxin due to high thujone content, and will cause medically dangerous spontaneous abortions in pregnant women
Mustard
Brassica nigra - mustard essential oil is considered to be one of the most toxic of all essential oils; an oral toxin, dermal (skin) toxin, and very irritating to mucous membranes (ex. the membranes inside your nose)
Narcissus
Narcissus poeticus - besides causing headaches and vomiting, Narcissus profoundly effects the nervous system and can
cause paralysis
Oregano, Common and Spanish
Origanum vulgare and Thymus capitatus, respectively - dermal (skin) toxin and irritant, and very irritating to mucous membranes (ex. the membranes inside your nose)
Pennyroyal
Mentha pulegium - an oral toxin that also causes medically dangerous spontaneous abortions in pregnant women
Pine, Dwarf
Pinus mugo var. pumilio - dermal (skin) irritant that causes sensitization (allergic reactions in people who do not have allergies)
Rue
Ruta graveolens - the essential oil is highly toxic, however homeopathic remedies are safe and very useful; specifically an oral toxin, dermal (skin) irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and causes medically dangerous spontaneous abortions in pregnant women
Sage, Common
Salvia officinalis - an oral toxin that causes medically dangerous spontaneous abortions in pregnant women; although they should be avoided in pregnancy, Spanish Sage and Clary Sage are otherwise non-toxic alternatives
Santolina
Santolina chamaecyparissus - an oral toxin
Sassafras
Sassafras albidum - highly toxic, highly irritant, carcinogenic (causes cancer), and causes medically dangerous spontaneous abortions in pregnant women
Savine
Juniperus sabina - an oral toxin and dermal (skin irritant), it is banned from sale in many countries because of its effect on circulation and for causing medically dangerous spontaneous abortions in pregnant women
Savory, Summer and Winter
Satureja hortensis and Satureja montana, respectively - dermal (skin) toxin and irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and causes medically dangerous spontaneous abortions in pregnant women
Tagetes
Tagetes minuta - strongly suspected of causing dermatitis and considered generally harmful
Tansy
Tanacetum vulgare - an oral toxin due to high thujone content, and causes medically dangerous spontaneous abortions in pregnant women
Tarragon
Artemisia dracunculus - the essential oil is moderately toxic due to methyl chavicol, but more importantly it is suspected to be carcinogenic (causes cancer) and must not be used during pregnancy
Thuja
Thuja occidentalis - the essential oil is highly toxic, however homeopathic remedies are safe and very useful; an oral toxin, to the extent of being poisonous, and causes medically dangerous spontaneous abortions in pregnant women
Thyme, Red, Serpolet (wild), and "thymol" and "carvacrol" types
Thymus vulgaris - the lemon and "linalol" types are generally less toxic and considered safe for use on the skin and with children (white thyme is often adulterated and should be avoided); contain large amounts of toxic phenols, dermal (skin) irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and must be avoided during pregnancy or if you have high blood pressure
Tonka
Dipteryx odorata - an oral and dermal (skin) toxin because of its coumarin content
Turpentine
Pinus palustris and other Pinus species labeled as "Turpentine" - turpentine essential oil is an environmental hazard and a marine pollutant
Wintergreen
Gaultheria procumbens - the essential oil is a toxin, irritant, and sensitizing; in fact, wintergreen essential oil is considered environmentally hazardous and a marine pollutant
Many retail stores sell wintergreen essential oil despite the safety issues - do not assume that just because you see it on the retail shelf that it is safe!
Wormseed
Chenopodium ambrosioides var. anthelminticum - a very toxic essential oil that can explode when heated due to ascaridole content
Wormwood
Artemisia absinthium - extremely toxic, and will cause medically dangerous spontaneous abortions in pregnant women
Credit for the list goes to ebay user freedomforhealth. (If he got this info from somewhere else i do not know.)
i hope this helps and good luck on your soap.