Ambient air quality guideline — particulate matter threshold As per World Health Organization (WHO) specifications, what is the maximum permissible concentration (TLV) for particulate matter in air, expressed in μg/m³?

Introduction / Context:
Airborne particulate matter is one of the most critical air pollutants because it affects human lungs, cardiovascular health, visibility, and overall environmental quality. Regulatory and advisory bodies publish threshold values to indicate safe or acceptable concentrations for public health. This question asks you to recall the World Health Organization (WHO) threshold limit value (TLV) magnitude traditionally referenced for particulates in ambient air.

Given Data / Assumptions:

• The quantity of interest is particulate matter concentration in μg/m³ (micrograms per cubic meter).
• The question seeks the standard magnitude historically cited as a permissible guideline figure.
• We are not distinguishing between size fractions (e.g., PM10 vs PM2.5) in the options; the intent is recognition of the order of magnitude for the WHO-type TLV used in classic MCQs.

Concept / Approach:
WHO guidance has evolved, but classic engineering MCQs often use a representative TLV magnitude near tens to low hundreds of μg/m³, not the thousands. The order of magnitude must reflect plausible ambient air guidance rather than occupational dust limits or stack emissions. Values such as 750 or 800 μg/m³ are extremely high for ambient compliance thresholds.

Step-by-Step Solution:

Verification / Alternative check:
Many standard environmental engineering texts present ambient particulate guidelines in the tens to low hundreds of μg/m³ for legacy questions, placing 90 μg/m³ in the expected range for a TLV-style figure.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing modern fine-particle (PM2.5) annual targets with older or total suspended particulate references; also mixing occupational exposure values with ambient guidelines.

Final Answer:
90