Cecil is very correct in that the book of revelation was written with heavy use of symbolism. You really have to study the scriptures around the one you are confused about in order to understand them.
There is also the translation to consider. My New World Translation renders the scripture in Revelation 8:7 as:
"The first one blew his trumpet. And there was hail and fire mingled with blood, and it was hurled to the earth; and a third of the earth was burned up, and a third of the trees were burned up, and all the green vegetation was burned up."
However those scriptures are talking about the same thing whether it be grass or vegetation. I have the book, but it is not currently on the website. I will be using the book Revelation-ITS GRAND CLIMAX AT HAND! chapters 21 and 22 as my source for gleaning more from the book of revelation. The book itself is very well researched and breaks down each scripture and explains it. So on the topic's we are looking at in chapters 8 and 9 it says this.
"In the Bible, the word “earth” often refers to mankind. (Genesis 11:1; Psalm 96:1) Since the second plague is on the sea, which also has to do with mankind, “the earth” must refer to the seemingly stable human society that Satan has built up and that is due to be destroyed. (2 Peter 3:7; Revelation 21:1) The plague scenario reveals that false religion's third of the earth is scorched by the searing heat of Jehovah’s disapproval. Her prominent ones—standing like trees in the midst of her—are burned up by the proclaiming of Jehovah’s adverse judgment. All her hundreds of millions of members, if they continue to support false religion, become like scorched blades of grass, spiritually wilted in God’s eyes.—Compare Psalm 37:1, 2."
Really then false religion is not being literally burned up, but spiritually. They are at that point experiencing God's full disapproval of them.
Later in chapter 9 of revelations that book again elaborates as to what those symbols mean.
"First, though, let us examine other scriptures that will help us to understand this plague better.
The Bible book of Joel, written during the ninth century B.C.E., describes a plague of insects, including locusts, that is similar to the one that John sees. (Joel 2:1-11, 25) It was to cause much discomfort for apostate Israel but would also result in individual Jews’ repenting and returning to Jehovah’s favor. (Joel 2:6, 12-14) When that time arrived, Jehovah would pour out his spirit upon “every sort of flesh,” while fearful signs and alarming portents would precede “the coming of the great and fear-inspiring day of Jehovah.”—Joel 2:11, 28-32.
"There was a fulfillment of Joel chapter 2 in the first century. It was then, at Pentecost 33 C.E., that holy spirit was poured out, anointing the first Christians and empowering them to speak “the magnificent things of God” in many tongues. As a result, a large crowd assembled. The apostle Peter addressed those astonished onlookers, quoting Joel 2:28, 29 and explaining that they were witnessing its fulfillment. (Acts 2:1-21) But there is no record of a literal insect plague at that time, causing discomfort to some and leading others to repentance."
"Was there a figurative plague during those days? Yes, indeed! It came as a result of the relentless preaching of the newly anointed Christians. Through them, Jehovah invited those Jews who would listen to repent and enjoy blessings from him. (Acts 2:38-40; 3:19) The individuals who responded received his favor to a remarkable degree. But to the ones who refused the invitation, the first-century Christians became like a devastating swarm of locusts. Starting in Jerusalem, they spread through all Judaea and Samaria. Soon they were everywhere, tormenting the unbelieving Jews by publicly proclaiming Jesus’ resurrection, with all that this implied. (Acts 1:8; 4:18-20; 5:17-21, 28, 29, 40-42; 17:5, 6; 21:27-30) That plaguing continued until the “fear-inspiring day,” in 70 C.E., when Jehovah brought the Roman armies against Jerusalem to destroy it. Only those Christians who in faith called on the name of Jehovah were saved.—Joel 2:32; Acts 2:20, 21; Proverbs 18:10."
So something very similar was prophesied about locusts tormenting people and an outcome is given in the bible. So what about the modern time fulfillment of revelations locusts? Revelation 9:4-6
"They were told not to harm the vegetation of the earth or any green plant or any tree, but only those people who do not have the seal of God on their foreheads. And it was granted the locusts, not to kill them, but to torment them five months, and their torment was like torment by a scorpion when it strikes a person. In those days people will seek death but will by no means find it, and they will long to die, but death will flee from them."
And the books explanation:
"Notice that this plague is not directed first against the people or prominent ones among them—the ‘vegetation and trees of the earth.’ (Compare Revelation 8:7.) The locusts are to harm only those men who do not have the seal of God on their foreheads, those of false religion who claim to be sealed but whose record belies that claim. (Ephesians 1:13, 14) Thus, the tormenting utterances of these modern-day locusts were directed first against the false religious leaders. How these self-assuming men must have been tormented at hearing it publicly announced that not only were they failing to lead their flocks to heaven but they themselves would not get there! Truly, it has been a case of ‘the blind leading the blind’!
So now that we know what those passages mean we can analyze your question better. The green vegetation or grass mentioned in chapter 9 are the men that are not in a leadership position be it government or religious. They are the common people.
Because revelations is very symbolic is is tough to say what portion of mankind is included in the grass/vegetation or what it means for them to be burnt up. That could mean a spiritual burning or withering. Thus they are spiritually weakened, but they can be hurt or weakened further.