Mustard oil
The term mustard oil is used for two different oils that are made from mustard seeds:
- a fatty vegetable oil resulting from pressing the seeds,
- an essential oil resulting from grinding the seeds, mixing them with water, and extracting the resulting volatile oil by distillation.

# Mustard oil from pressed seeds
This oil has a strong smell, a little like strong cabbage, a hot nutty taste, and is much used for cooking in Orissa, Bengal, Bihar and other areas of India and Bangladesh. In north India, it is mainly used in frying fritters. The oil makes up about 30% of the mustard seeds.  It can be produced from black mustard (Brassica nigra), brown Indian mustard (Brassica juncea), and white mustard (Brassica hirta).
Mustard oil is composed mostly of the fatty acids oleic acid, linoleic acid and erucic acid. At 5%, mustard seed oil has the lowest saturated fat content of the edible oils.
In India, mustard oil is generally heated almost to smoking before it is used for cooking; this may be an attempt to reduce the content of noxious substances such as erucic acid, and does reduce the strong smell and taste.  , Mustard oil is not considered suitable for human consumption in the United States, Canada and the European Union due to the high content of erucic acid, which is considered noxious, although mustard oil with low erucic acid content is available. To get around the restriction in Western countries, the oil is often sold "for external use only" in stores catering to Indian immigrants, as in North India, mustard oil is also used for rub-downs and massages (see ayurveda), thought to improve blood circulation, muscular development and skin texture; the oil is also antibacterial.
In India, the restrictions on mustard oil are viewed as an attempt by foreign multi-national corporations to replace mustard oil with canola oil, a variety of rapeseed with a low erucic acid content, but often from a genetically modified canola. The East and North Indians have been using it for ages and deny that there is enough evidence for the toxicity of erucic acid, instead maintaining that mustard oil is beneficial to human health because of its low  saturated fat content, ideal ratio of omega-3 and omega 6 fatty acids (15g of omega 3 fats per 100g serving), content of antioxidants and vitamin E, as well as being cold-pressed (extracted at 45-50 degrees Celsius).
In northern Italy, it is used in the fruit condiment called mostarda.

# Mustard oil from mixing seeds with water
The pungency of the condiment mustard results when ground mustard seeds are mixed with water, vinegar, or other liquid (or even when chewed). Under these conditions, a chemical reaction between the enzyme myrosinase and a glucosinolate known as sinigrin from the seeds of black mustard (Brassica nigra) or brown Indian mustard (Brassica juncea) produces allyl isothiocyanate. By distillation one can produce a very sharp-tasting essential oil, sometimes called volatile oil of mustard, containing more than 92% allyl isothiocyanate. The pungency of allyl isothiocyanate is due to the activation of the TRPA1 ion channel in sensory neurons. White mustard Brassica hirta does not yield allyl isothiocyanate, but a different and milder isothiocyanate.
Allyl isothiocyanate serves the plant as a defense against herbivores. Since it is harmful to the plant itself, it is stored in the harmless form of a glucosinolate, separate from the myrosinase enzyme. Once the herbivore chews the plant, the noxious allyl isothiocyanate is produced. Allyl isothiocyanate is also responsible for the pungent taste of horseradish and wasabi. It can be produced synthetically, sometimes known as synthetic mustard oil.
Because of the contained allyl isothiocyanate, this type of mustard oil is toxic and irritates the skin and mucous membranes. In very small amounts, it is often used by the food industry for flavoring. It is also used to repel cats and dogs.  It will also denature alcohol, making it unfit for human consumption, thus avoiding the taxes collected on alcoholic beverages.
The CAS number of this type of mustard oil is 8007-40-7, and the CAS number of pure allyl isothiocyanate is 57-06-7.

# Use of mustard oil in North Indian cultural activities
Mustard oil, though not very popular as a cooking oil in North India, still is intricately embedded in the culture, used in these contexts:
- It is poured on both sides of threshold when someone important comes home for the first time (e.g. a newly-wedded couple or a son or daughter when returning after a long absence, or succeeding in an exams or election.
- Used as traditional jaggo pot fuel in Punjabi weddings.
- Used as part of home-made cosmetics during mayian.