Formula
Mathematically:

  
    
      
        p
        =
        
          
            F
            A
          
        
        ,
      
    
    {\displaystyle p={\frac {F}{A}},}
  where:

  
    
      
        p
      
    
    {\displaystyle p}
   is the pressure,

  
    
      
        F
      
    
    {\displaystyle F}
   is the magnitude of the normal force,

  
    
      
        A
      
    
    {\displaystyle A}
   is the area of the surface on contact.Pressure is a scalar quantity.
The main concern about smoking and breastfeeding is that infants may have smoking-induced reductions to the milk iodine content. Smoking can adversely affect the lactation process by decreasing milk production and altering the milk composition. Smoking reduces daily milk output by roughly 250–300 mL. Not only will this be problematic on a daily basis for not producing enough milk, it will also cause the mother to wean her baby early. The altered milk composition also caused infants to exhibit daily behaviors such as colic and crying which can promote early weaning, again something that is not beneficial to the infant.Also, the nicotine obtained from smoking travels through a woman into her breast milk, thus giving nicotine to her child.Heavy use of cigarettes by the mother (more than 20 per day) has been shown to reduce the mothers milk supply and cause vomiting, diarrhoea, rapid heart rate, and restlessness in breastfed infants. Sudden Infant Death Syndrome (SIDS) is more common in babies exposed to a smoky environment. Breastfeeding mothers who smoke are counseled not to do so during or immediately before feeding their child, and are encouraged to seek advice to help them reduce their nicotine intake or quit. Other substance abuse
With respect to alcohol, the American Academy of Pediatrics states that when breastfeeding, "moderation is definitely advised" and recommends waiting for 2 hours after drinking before nursing or pumping.