I think I should provide some more input..
If you are on the ground (pre-flight planning) and the sky looks blue and clear, high cloud and sun shining. You check the METARS and TAFS..
The reports come back as 'XXxx 281000Z 24006KT CAVOK 27/15 Q1011'
TAF comes in at something like ' XXXX ... 26007KT 9999 FEW 045 BECMG 2817/28/20 01010KT
Haze doesn't get reported in a METAR (I believe).. Mist ('BR') does hence in this case it is clear cut you wouldn't go up because it is not legal.
On the ground, you only have these METAR/TAF reports and a few other sources like Rain Radar, Weather Online, XC Weather etc.. to go by to make a 'go or no go decision' - until you get into the SKY... and then it turns out to be Hazy - as was the case on my Skills Test under the Examiners License! Going back a a short while ago!
What do you do - if you are below minimums I agree it is illegal. Once should in theory turn back and land.. I don't know if PPL without IMC rating or CPL without IR actually turn back or not?
But if you believe according to your 'eyes' that the vis is 5km and you are legal and it is just 'hazy' - what actions are helpful to conduct to make it a safer flight? Climb, Descend etc
Scoobster