Better resolution with photodiode (LTR-536AD)
You need to measure only the change in some property caused by your "fog". This typically is a change in intensity, converted to a voltage by a photodiode and a transimpedance amplifier and measured.  You're sensing changes in \$\mu_s\$ (scattering coefficient) which results in changes in intensity.

You need to eliminate any effect not caused by the "fog" - change in ambient lighting etc.  

Some suggestions are:


Chose a wavelength where intensity of the detected beam is maximally changed (@Janka correctly suggests a short bluish wavelength - but this may not be possible as sources of blue laser light are more expensive) 
Filter out light that is not of the above wavelength - if you're using blue light, any red light at the detector is noise and needs to be rejected. Use an optical filter to eliminate it. The narrower the better, but narrowband filters are expensive. (Need to make sure that temperature change in the laser doesn't change the wavelngth to wander out of the passband.
Modulate the light. This allows you to ac couple the amplifier and now dc offsets are not a problem. (Don't want to measure a small change on top of a large offset) When you detect the light use a lock in amplifier and you can narrow the detection bandwidth (ie lots of averaging) so your SNR improves as the BW decreases.  You can modulate a frequency which is not related to the mains power and can eliminate interference from room lighting. Avoids \$1/f\$ noise in the detector.
Alternate between signal and background. Measure your detection background for a second, turn your laser on then measure again for a second.
Maximize your signal. Use several mirrors to make the path length thru the fog as long a possible.
Make you light source a bright as possible. Then you are well away from thermal noise sources in the detecting circuitry. You want to be shot noise limited.
Collimate the light from the laser - use a lens to take that parallel rays onto the photodiode - to make the angular field of view (what the photodiode sees) only that of the source.
Put it in a box where the fog can enter but EMI or stray light cannot. (Just like a smoke detector)
Make everything solid.  The system shouldn't be sensitive to temperature, vibration etc.