Air tries to equalize between higher and lower air pressure areas. If there is a pathway (a gap) and a pressure difference, air will move through that pathway - whether you want it to or not.

• To be an effective air barrier or air flow retarder, a material must both resist air pressure differences and be installed in a way that eliminates even small holes or gaps.

If the amount of air removed from an enclosed space (exhaust systems or air return to the air conditioner) does not equal the amount of air supplied to that space, then a pressure imbalance is created.

• A negative pressure happens when more air is removed than added, causing the building to draw in make-up air (air infiltration).
• Leaky ducts in attics cause negative pressure inside homes and increase infiltration of air through walls, ceilings and floors. In summer, that means hot, humid air infiltration.
• A large-enough negative pressure can lead to dangerous backdrafting of the chimneys and flues of combustion appliances.
  
• A positive pressure happens when more air is supplied than removed from a space.
• A positive pressure can be created and controlled with a fresh air intake.
• A strong wind (hurricane) entering a home through a broken window can pressurize the house enough to damage it.
• Forces can create simultaneous positive and negative pressures in different parts of a building. An example is closing a door to a room that has supply registers delivering air but no return grille, making the room positive and the area with the return grille negative.

Indoor air typically contains more types and higher concentrations of pollutants than outdoor air, even in industrialized areas.

• Common home indoor air pollutants include biological pollutants (mold spores, dust mites, bacteria, viruses, pollen, animal dander), combustion pollutants (including carbon monoxide), lead in dust (from old paint or lead-tainted soil), volatile organic compounds (VOCs) from out-gassing chemicals and materials, and sometimes asbestos. Less common in Louisiana homes is radon, a radioactive soil gas.

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