Foul smell usually arise from rotting material usually organic, either dead tissue or mucus or something collected inside the nostrils. If there is a constant foul smell especially perceptible to others then we should get it checked. There are various reasons for this to happen. Especially in case of children they have a habit of pushing something into the nostrils. That can sit inside the nostril and create a reaction around it and if it is an organic material like a seed then it will also rot along with the mucus collected around it. That will cause foul smell and profuse mucopurulent or pus discharge from only one side. Get the baby to the doctor and get it checked. The material has to be removed and proper treatment administered. In adults it is indigenous, from inside the nose. The mucus becomes very thick and stagnant rather than the usual one which moves backwards and gets swallowed as a routine. If there is a problem in the mobility of the mucus it starts getting collected inside the nose. There are other conditions like atrophic rhinitis or rhinitis sicca where the character of the mucus changes and the smell comes up. In atropic rhinitis the patient’s will not feel the smell because they will lose the sensation and the others could feel it and they would bring the patient to the doctor. Anyway it is a rotting material and it has to be removed. In case of a long term disease the care has to be taken on a long term and regular cleaning required with saline or something. The ancient practice of jal neethi practised by yoga practitioners can also be helpful in removing this material.