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In a study conducted in 117 countries, researchers have found that the world is experiencing the most dramatic reduction in the seismic noise (the hum of vibrations in the planet's crust) in recorded history due to global COVID-19 lockdowns.

Measured by instruments called seismometers, seismic noise is caused by vibrations within the Earth, which travel like waves and the waves can be triggered by earthquakes, volcanoes, and bombs - but also by daily human activity like travel and industry.

This quiet period was likely caused by the total global effect of social distancing measures, closure of services and industry, and drops in tourism and travel, the study published in the journal Science, reported.

The new research, led by the Royal Observatory of Belgium and five other institutions around the world including Imperial College London (ICL), showed that the dampening of 'seismic noise' caused by humans was more pronounced in more densely populated areas.

"Our study uniquely highlights just how much human activities impact the solid Earth, and could let us see more clearly than ever what differentiates human and natural noise," said study co-author Stephen Hicks from ICL in the UK.

For the findings, the research team looked at seismic data from a global network of 268 seismic stations in 117 countries and found significant noise reductions compared to before any lockdown at 185 of those stations.

Researchers tracked the 'wave' of quietening between March and May as worldwide lockdown measures took hold.

The largest drops in vibrations were seen in the most densely populated areas, like Singapore and New York City, but drops were also seen in remote areas like Germany's the Black Forest and Rundu in Namibia.

Citizen-owned seismometers, which tend to measure more localised noise, noted large drops around universities and schools around Cornwall, UK and Boston, US - a drop in noise 20 per cent larger than seen during school holidays.

The findings showed that countries like Barbados, where lockdown coincided with the tourist season, saw a 50 per cent decrease in noise.

"The changes have also given us the opportunity to listen in to the Earth's natural vibrations without the distortions of human input," the study authors wrote.

Earlier in April, a study published in the journal Nature, reported at least a 30 per cent reduction in that amount of ambient human noise since lockdown began in Belgium.

In the wake of the gas leak at a factory in Visakhapatnam, the National Disaster Management Authority (NDMA) has issued detailed guidelines for restarting industries after the lockdown and the precautions to be taken for the safety of the plants as well as the workers.

In a communication to all states and union territories, the NDMA said due to several weeks of lockdown and the closure of industrial units, it is possible that some of the operators might not have followed the established standard operating procedures.

As a result, some of the manufacturing facilities, pipelines, valves may have residual chemicals, which may pose risk. The same is true for the storage facilities with hazardous chemicals and flammable materials, it said.

The NDMA guidelines said while restarting a unit, the first week should be considered as the trial or test run period after ensuring all safety protocols.

Companies should not try to achieve high production targets. There should be 24-hour sanitisation of the factory premises, it said.

The factories need to maintain a sanitisation routine every two-three hours especially in the common areas that include lunch rooms and common tables which will have to be wiped clean with disinfectants after every single use, it added.

For accommodation, the NDMA said, sanitisation needs to be performed regularly to ensure worker safety and reduce the spread of contamination.

To minimise the risk, it is important that employees who work on specific equipment are sensitised and made aware of the need to identify abnormalities like strange sounds or smell, exposed wires, vibrations, leaks, smoke, abnormal wobbling, irregular grinding or other potentially hazardous signs which indicate the need for immediate maintenance or if required shutdown, it said.

At least 11 people lost their lives and about 1,000 others were exposed to a gas leak at a factory in Andhra Pradesh''s Visakhapatnam on May 7.

The incident took place after it restarted operations when the government allowed industrial activities in certain sectors following several weeks of lockdown.

The lockdown was first announced by Prime Minister Narendra Modi on March 24 for 21 days in a bid to combat the coronavirus threat. The lockdown was then extended till May 3 and again till May 17.

Denser places, assumed by many to be more conducive to the spread of the coronavirus that causes COVID-19, are not linked to higher infection rates, say researchers.

The study, led by Johns Hopkins University, published in the Journal of the American Planning Association, also found that dense areas were associated with lower COVID-19 death rates.

"These findings suggest that urban planners should continue to practice and advocate for compact places rather than sprawling ones, due to the myriad well-established benefits of the former, including health benefits," says study lead author Shima Hamidi from Johns Hopkins Bloomberg School of Public Health in the US.

For their analysis, the researchers examined SARS-CoV-2 infection rates and COVID-19 death rates in 913 metropolitan counties in the US.

When other factors such as race and education were taken into account, the authors found that county density was not significantly associated with county infection rate.

The findings also showed that denser counties, as compared to more sprawling ones, tended to have lower death rates--possibly because they enjoyed a higher level of development including better health care systems.

On the other hand, the research found that higher coronavirus infection and COVID-19 mortality rates in counties are more related to the larger context of metropolitan size in which counties are located.

Large metropolitan areas with a higher number of counties tightly linked together through economic, social, and commuting relationships are the most vulnerable to the pandemic outbreaks.

According to the researchers, recent polls suggest that many US citizens now consider an exodus from big cities likely, possibly due to the belief that more density equals more infection risk.

Some government officials have posited that urban density is linked to the transmissibility of the virus.

"The fact that density is unrelated to confirmed virus infection rates and inversely related to confirmed COVID-19 death rates is important, unexpected, and profound," said Hamidi.

"It counters a narrative that, absent data and analysis, would challenge the foundation of modern cities and could lead to a population shift from urban centres to suburban and exurban areas," Hamidi added.

The analysis found that after controlling for factors such as metropolitan size, education, race, and age, doubling the activity density was associated with an 11.3 per cent lower death rate.

The authors said that this is possibly due to faster and more widespread adoption of social distancing practices and better quality of health care in areas of denser population.

The researchers concluded that a higher county population, a higher proportion of people age 60 and up, a lower proportion of college-educated people, and a higher proportion of African Americans were all associated with a greater infection rate and mortality rate.