A bad air filter presents lots of trouble for the car. First, if the air is not filtered as it should be, and dirt and debris reach the engine, that might cause damage to the engine; sometimes the damage can be substantial or irreparable.
Again, a bad air filter prevents adequate clean air from reaching the engine, and as a result, the car’s emission control system can be affected. That leads to the emission of black, sooty exhaust gases.
As the fuel-air mixture contains too much fuel and too little air, the spark plugs might also get damaged.