The use of chemicals such as formaldehyde and asbestos has decreased because they have been

The use of chemicals such as formaldehyde and asbestos has decreased because they have been potentially cancerogenous.  

Formaldehyde is the gas emitted from certain products that we have in homes (plywood and panelling, furniture), which can cause watery eyes, burning sensations in the eyes and throat, nausea, and difficulty breathing in some people. Some of the recent studies have shown a positive correlation between exposure to formaldehyde and the development of leukemia (carcinoma of white blood cells).

Asbestos is a mineral fibre often found in the home can also become an air pollutant. Breathing high levels of asbestos fibres (which are released when the asbestos is damaged) over a long period can lead to an increased risk of lung cancer or some other respiratory diseases.

Homologous chromosomes with heterozygous alleles d) heterozygous chromosomes with homozygous alleles