What are the reasons of Oder inside the building? and how could we control it?

odor inside building can be because of various reasons

Poor exhaust from the kitchen or toilets.

The fresh in take may have some odour source near by

A died mouse or rat inside duct system.

A hole or opening in the shaft near return plenum of AC units 

Lack of Fresh in take

poor venting system in drainage

ODOR SOURCES

Outdoor sources:

include automotive and diesel exhausts, hazardous waste sites, sewage treatment plants, compost piles, refuse facilities, printing plants, refineries, chemical plants, and many other stationary and mobile sources. These sources produce both inorganic compounds (e.g., ammonia and hydrogen sulfide) and volatile organic compounds (VOCs), including some that evaporate from solid or liquid particulate matter. Odors emitted by outdoor sources eventually enter the indoor environment.

Indoor sources:

Sources include tobacco products, bathrooms and toilets, building materials (adhesives, paints, caulks, processed wood, carpets, plastic sheeting, and insulation board), consumer products (e.g., food, toiletries, cleaning materials, and polishes), hobby materials, fabrics, and foam cushions. In offices, offset printing processes, copiers, and computer printers may produce odors. Electrostatic processes may emit ozone.

Humans emit a wide range of odorants, including acetaldehyde, ammonia, ethanol, hydrogen sulfide, and mercaptans.

for sure providing good ventilation will control the Odor but The ability of ventilation to control odors also depends on the strength of the source generating the odorant(s) and the nature of the odor. An odorant with a stronger source requires proportionately more ventilation to achieve the same reduction in concentration. Odors that are perceived as unpleasant may require substantially greater reduction before being perceived as acceptable. In addition, some sources, such as painted walls and flooring materials, may show increased emission rates in response to increased ventilation rates, which further complicates the issue (Gunnarsen 1997)

2005 ASHRAE Handbook—Fundamentals (SI) CH. 13

many reasons:

1. connection of condensate water with sewage (especially when it is poorly ventilated) => we must evacuate the condensat separately=> we must properly ventilate the drainage network system to eliminate reverse phenomena.

2. Drainage: poor ventilation of catch basin and manhole etc. 

3. Lack of air exchange in the air conditioning system installed => it is necessary to carefully calculate the flow of fresh air  depending on the application, N.B: we must properly size a regulation system for the hummidity Control.

4. Lack of fresh air filtration, as well as recirculated air

5. Lack of cleaning of filters and lack of maintenance

6. uncontrolled Hummidity 

 => The fresh air system involves a technical installation in the building in order to import and filter the outside air, wants to be the best alternative for accommodation in the quality of air. A notion which, in addition to controlling the humidity, a stable temperature, minimizing the risk of allergies and odor removal.

etc. 

your question about the odor:

the reasons may be from WC and kitchen, if we select the exhaust fan with suitable exhaust fan we can reduce it,