What is the maximum permissible limit of lead in drinking water in $ppb$?

Which pollutants are present in the troposphere?

The chemical added to leaded petrol to prevent the deposition of lead in the combustion chamber is

Which one of the following gases converts haemoglobin into carboxyhaemoglobin?

Fill in the blanks:
$(1)$ Deficiency of fluoride is responsible for ............. .
$(2)$ The upper limit of lead concentration in drinking water is ............. .
$(3)$ The maximum limit of nitrate in drinking water is ............. .
$(4)$ ............. and ............. chemicals act as poisons for the nervous system.

The depletion of the ozone layer in the stratosphere is responsible for which of the following?